UDP performance

[splitice]

Use recvmmsg instead of recvfrom for udp message receiving for improved performance.

I suggest using socket2 alongside the code in this issue rust-lang/socket2#493, replace the existing udp implementation for higher performance on linux. With the easiest way to be using this fork https://github.com/boris-n/socket2/tree/sendmmsg-recvmmsg

On operating systems where recvmmsg is not supported continue to use the current implementation.

See also:

• https://github.com/sebastiencs/rustorrent/blob/6829a99fb94a4badc948533f4fc75a9f0e4a32c2/src/utp/udp_socket.rs
• Improve Vector's throughput for UDP protocols vectordotdev/vector#8518