release-2.1: storage: eagerly queue undersize ranges for merging

[benesch]

Backport 1/1 commits from #30590.

/cc @cockroachdb/release

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When processing a write, check if the range has dropped beneath the
minimum size threshold. If it has, check whether the merge queue would
like to process it. This is analogous to our logic for eagerly splitting
oversize ranges.

Fix #29004.

Release note: None

Before (courtesy of @tschottdorf):

After:

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

This change is 

[tschottdorf]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained

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pkg/storage/replica_proposal.go, line 537 at r1 (raw file):

	rResult.Delta = enginepb.MVCCStatsDelta{}

	if r.store.splitQueue != nil && needsSplitBySize { // the boostrap store has a nil split queue

boostrap here and below (but if you bother changing it, let's do it on master and then backport.. so I don't think you should bother)

[benesch]

Reviewable status: complete! 0 of 0 LGTMs obtained

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pkg/storage/replica_proposal.go, line 537 at r1 (raw file):

Previously, tschottdorf (Tobias Schottdorf) wrote…

boostrap here and below (but if you bother changing it, let's do it on master and then backport.. so I don't think you should bother)

D'oh! Thanks.