colblk: cache IndexBlockDecoder in block metadata

[RaduBerinde]

RandSeekInSST/v4/single-level-10  1.21µs ± 1%  1.25µs ± 5%   +3.40%  (p=0.005 n=8+8)
RandSeekInSST/v4/two-level-10     2.06µs ± 4%  2.06µs ± 1%     ~     (p=0.959 n=8+8)
RandSeekInSST/v5/single-level-10  1.11µs ± 1%  1.06µs ± 1%   -4.03%  (p=0.000 n=8+8)
RandSeekInSST/v5/two-level-10     1.79µs ± 5%  1.59µs ± 1%  -11.45%  (p=0.000 n=7+8)

[cockroach-teamcity]

This change is 

[jbowens]

Reviewed 6 of 7 files at r1, all commit messages.
Reviewable status: 6 of 7 files reviewed, 1 unresolved discussion (waiting on @RaduBerinde)

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sstable/colblk/index_block.go line 229 at r1 (raw file):

	i.h.Release()
	i.h = block.BufferHandle{}
	i.allocDecoder.Init(blk)

Now that we expect most instances to use a pre-allocated IndexBlockDecoder, should we remove i.allocDecoder and update callers of IndexIter.Init to use InitWithDecoder?

[RaduBerinde]

sstable/colblk/index_block.go line 229 at r1 (raw file):

Previously, jbowens (Jackson Owens) wrote…

Now that we expect most instances to use a pre-allocated IndexBlockDecoder, should we remove i.allocDecoder and update callers of IndexIter.Init to use InitWithDecoder?

There are quite a few uses and I'm not sure if any are performance sensitive. Will leave a TODO for now.

[RaduBerinde]

TFTR!