[Rhythm] Block-builder improvement to skip WAL

[mdisibio]

What this PR does:
Update the block-builder to skip the WAL and go directly from in-memory live traces to the backend block. This is more efficient and the block-builders have much higher throughput. Memory allocs are improved, but total working must now be large enough to contain the full cycle of data. Overall the higher throughput is the most important for operational reasons.

               │ before.txt  │              after.txt              │
               │   sec/op    │   sec/op     vs base                │
BlockBuilder-4   808.0m ± 1%   374.7m ± 2%  -53.63% (p=0.000 n=10)

               │  before.txt  │               after.txt               │
               │     B/s      │     B/s       vs base                 │
BlockBuilder-4   36.17Mi ± 1%   78.41Mi ± 1%  +116.80% (p=0.000 n=10)

               │  before.txt   │              after.txt               │
               │     B/op      │     B/op      vs base                │
BlockBuilder-4   1165.5Mi ± 2%   839.9Mi ± 2%  -27.94% (p=0.000 n=10)

               │ before.txt  │             after.txt              │
               │  allocs/op  │  allocs/op   vs base               │
BlockBuilder-4   5.773M ± 0%   5.417M ± 0%  -6.17% (p=0.000 n=10)

Other changes:

• Integrate max trace size check into liveTraces. Block-builders don't track trace sizes across cycles so don't need traceSizes and reduces map overhead.
• Validate block-builder config on startup.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[mapno]

I like this changes, but I have mixed feelings about the live traces iterator. It's nondeterministic too much, makes it less predictable.