Add batch multi-get execution for improved performance

[bisho]

Optimize multi-get operations by using batch socket API

This change improves the performance of multi-get operations by implementing a dedicated _exec_multi_get_on_pool method that uses the socket's meta_multiget batch API instead of executing individual get commands in a loop.

Changes:

• Added _exec_multi_get_on_pool method to handle META_GET commands efficiently using the batch API
• Modified exec_multi_on_pool to route META_GET commands to the new optimized path
• Updated test mocks to use meta_multiget instead of individual send_meta_get/get_response calls
• Enhanced test coverage to verify proper flag passing and batch operation behavior

This optimization reduces the number of socket operations and improves throughput for multi-get requests while maintaining the same error handling and response processing behavior.

Performance

Common config: server 127.0.0.1:11211, 10 threads, 1.00M requests (4 runs × 25,000 ops × 10 threads), 200 key space, GC disabled, 160/200 keys populated (95% small 80-250B, 5% large 100KB)                                                                                

  ┌───────────────┬────────┬─────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┐                                                                                                                                                                
  │               │        │         │    Socket in Rust    │    meta_* in Rust    │   key-enc in Rust    │   multiget in Rust   │
  │   Operation   │        │ BEFORE  ├──────────┬───────────┼──────────┬───────────┼──────────┬───────────┼──────────┬───────────┤                                                                                                                                                                
  │               │        │         │  Result  │  Speedup  │  Result  │  Speedup  │  Result  │  Speedup  │  Result  │  Speedup  │                                                                                                                                                                
  ├───────────────┼────────┼─────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┤                                                                                                                                                                
  │               │ RPS    │  46,600 │   76,652 │           │  110,287 │           │  112,054 │           │  111,638 │           │                                                                                                                                                                
  │  SET          ├────────┼─────────┼──────────┤   1.65x   ├──────────┤   2.37x   ├──────────┤   2.40x   ├──────────┤   2.40x   │                                                                                                                                                                
  │               │ us/req │   21.46 │    13.05 │           │     9.07 │           │     8.92 │           │     8.96 │           │                                                                                                                                                                
  ├───────────────┼────────┼─────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┤                                                                                                                                                                
  │               │ RPS    │  46,135 │   81,401 │           │  116,382 │           │  118,682 │           │  118,351 │           │                                                                                                                                                                
  │  GET          ├────────┼─────────┼──────────┤   1.76x   ├──────────┤   2.52x   ├──────────┤   2.57x   ├──────────┤   2.57x   │                                                                                                                                                                
  │               │ us/req │   21.68 │    12.28 │           │     8.59 │           │     8.43 │           │     8.45 │           │                                                                                                                                                                
  ├───────────────┼────────┼─────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┤                                                                                                                                                                
  │               │ RPS    │   2,677 │    3,626 │           │    3,560 │           │    3,567 │           │   14,118 │           │                                                                                                                                                                
  │  MULTI_GET(5) ├────────┼─────────┼──────────┤   1.35x   ├──────────┤   1.33x   ├──────────┤   1.33x   ├──────────┤   5.27x   │                                                                                                                                                                
  │               │ us/req │  373.52 │   275.81 │           │   280.86 │           │   280.31 │           │    70.83 │           │                                                                                                                                                                
  ├───────────────┼────────┼─────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┼──────────┼───────────┤                                                                                                                                                                
  │               │ RPS    │  48,080 │   82,886 │           │  123,053 │           │  125,639 │           │  127,131 │           │                                                                                                                                                                
  │  DELETE       ├────────┼─────────┼──────────┤   1.72x   ├──────────┤   2.56x   ├──────────┤   2.61x   ├──────────┤   2.64x   │                                                                                                                                                                
  │               │ us/req │   20.80 │    12.06 │           │     8.13 │           │     7.96 │           │     7.87 │           │                                                                                                                                                                
  └───────────────┴────────┴─────────┴──────────┴───────────┴──────────┴───────────┴──────────┴───────────┴──────────┴───────────┘

[bisho]

• Add batch multi-get execution for improved performance #83 👈 (View in Graphite)
• Make key encoder optional and remove default implementation #82
• Replace command building with direct socket method calls #81
• Migrate MemcacheSocket implementation to Rust #80
• Enhance benchmark with comprehensive SET/GET/DELETE testing #79
• main

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