[witness gene] optimize StepRecord only copy once

[hero78119]

design rationales

previous flow: tracer generate steprecord -> dispatch steprecord to respective witness table
now: tracer generate steprecord -> dispatch steprecord index only

tested on 23817600 generate_witness

Shard	master (s)	optimized (s)	Improvement (%)
0	6.88	4.13	39.97%
1	3.97	2.83	28.72%
2	3.72	2.85	23.39%
3	3.66	2.83	22.68%
4	2.80	2.19	21.79%
5	2.37	1.99	16.03%
6	4.34	4.42	-1.84%
7	6.40	3.78	40.94%
8	5.94	3.76	36.64%
9	5.69	3.83	32.69%
10	5.78	3.85	33.38%
11	3.80	2.72	28.42%
12	3.57	2.66	25.49%
13	3.51	2.59	26.21%
14	—	1.28	—