release-21.2: kvserver: don't use ClearRange point deletes with estimated MVCC stats

[blathers-crl]

Backport 1/1 commits from #74674 on behalf of @erikgrinaker.

/cc @cockroachdb/release

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ClearRange avoids dropping a Pebble range tombstone if the amount of
data that's deleted is small (<=512 KB), instead dropping point
deletions. It uses MVCC statistics to determine this. However, when
clearing an entire range, it will rely on the existing range MVCC stats
rather than computing them.

These range statistics can be highly inaccurate -- in some cases so
inaccurate that they even become negative. This in turn can cause
ClearRange to submit a huge write batch, which gets rejected by Raft
with command too large.

This patch avoids dropping point deletes if the statistics are estimated
(which is only the case when clearing an entire range). Alternatively,
it could do a full stats recomputation in this case, but entire range
deletions seem likely to be large and/or rare enough that dropping a
range tombstone is fine.

Resolves #74686.

Release note (bug fix): Fixed a bug where deleting data via schema
changes (e.g. when dropping an index or table) could fail with a
"command too large" error.

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Release justification: fixes a bug that can break index backfills.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is