Reuse buffers in ServerMessage<BsatnFormat>

[Centril]

Description of Changes

Fixes #2824.

Defines a global pool BsatnRowListBuilderPool which reclaims the buffers of a ServerMessage<BsatnFormat> and which is then used when building new ServerMessage<BsatnFormat>s.

Notes:

1. The new pool BsatnRowListBuilderPool reports the same kind of metrics to prometheus as PagePool does.
2. BsatnRowListBuilder now works in terms of BytesMut.
3. The trait method fn to_bsatn_extend is redefined to be capable of dealing with BytesMut as well as Vec<u8>.
4. A trait ConsumeEachBuffer is defined from ServerMessage<BsatnFormat> and down to extract buffers. <ServerMessage<_> as ConsumeEachBuffer>::consume_each_buffer(...) is then called in messages::serialize(...) just after bsatn-encoding the entire message and before any compression is done. This is the place where the pool reclaims buffers.

Benchmarks

Benchmark numbers vs. master using cargo bench --bench subscription -- --baseline subs on i7-7700K, 64GB RAM:

footprint-scan          time:   [21.607 ms 21.873 ms 22.187 ms]
                        change: [-62.090% -61.438% -60.787%] (p = 0.00 < 0.05)
                        Performance has improved.

full-scan               time:   [22.185 ms 22.245 ms 22.324 ms]
                        change: [-36.884% -36.497% -36.166%] (p = 0.00 < 0.05)
                        Performance has improved.

The improvements in footprint-scan are mostly thanks to #2918, but 7 ms of the improvements here are thanks to the pool. The improvements to full-scan should be only thanks to the pool.

API and ABI breaking changes

None

Expected complexity level and risk

2?

Testing

• Tests for Pool<T> also apply to BsatnRowListBuilderPool.

[joshua-spacetime]

Can you make the first commit its own independent PR?

[Centril]

Can you make the first commit its own independent PR?

Will do 👍

not applicable