Allocation pool the chunked iters in InstanceEnv

[Centril]

Description of Changes

This adds a ChunkPool which ChunkedWriter uses.
Also removes some dead code in InstanceEnv.

Based on flame graphs and timings, I'm not sure this changes much :/
We might want to try a different approach that doesn't do any chunking at all but rather writes to a singe buffer that we pool.
We can then keep an offset when iterating instead. Not sure how this will work with row boundaries though.

I measured things again for .iter(), and in this case, it seems like the time spent in datastore_table_scan_bsatn decreased by a few %. Not a huge amount, but still an improvement. The index scan bit not improving makes sense -- there's a unique index there and only a single element is fetched.

Fixes #2020.

[coolreader18]

LGTM. Do you know if we have any tests that actually exercise the chunk.len() > ROW_ITER_CHUNK_SIZE codepath? That feels like it'd be good to have if not especially as this gets optimized more.

[Centril]

Do you know if we have any tests that actually exercise the chunk.len() > ROW_ITER_CHUNK_SIZE codepath? That feels like it'd be good to have if not especially as this gets optimized more.

No sorry, I can neither confirm nor deny the existence of such tests. 😊