feat(wal): Benchmark and improve WAL writes using Reset.

[cyriltovena]

What this PR does / why we need it:

This adds a benchmark and some improvement to the write path of wal segment.

Which issue(s) this PR fixes:
Fixes https://github.com/grafana/loki-private/issues/1005 https://github.com/grafana/loki-private/issues/1004

❯ benchstat before.txt after.txt                                                                                                                                 
name       old time/op    new time/op    delta
Writes-16    11.9ms ±29%     6.5ms ± 2%  -44.84%  (p=0.008 n=5+5)

name       old alloc/op   new alloc/op   delta
Writes-16    12.9MB ± 9%     0.1MB ± 1%  -98.84%  (p=0.008 n=5+5)

name       old allocs/op  new allocs/op  delta
Writes-16     2.36k ±19%     0.30k ± 0%  -87.13%  (p=0.008 n=5+5)

[benclive]

Do we have to pass a ctx to the index? I think we can remove it because we only ever pass context.TODO(). We also don't pass a context to the parent SegmentWriter so the utility is very limited.

[cyriltovena]

Yes I'll look into that.

[benclive]

Should a streamSegment have a reset() method?
It looks like only one line to reset them but it would maintain consistency with our other Readers/Writers.

[benclive]

LGTM! Just one comment on the tests but the benchmark should already cover it so happy to approve regardless.

[benclive]

Could you add/modify this test case so the re-use case adds more data than the initial case?
If we had re-use errors, we'd likely panic when re-using a small buffer buit we'd be less likely to panic when re-using a large buffer.

[benclive]

This benchmark isn't running in parallel (since dst is reused). You could remove the pool for writers from the benchmark, or add a pool for dst and run the whole thing in parallel :)