release-23.1: storage: CheckSSTConflict fix for nexting over overlapping points

[blathers-crl]

Backport 1/1 commits from #99141 on behalf of @itsbilal.

/cc @cockroachdb/release

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Previously, the nexting logic around both iterators being at a range key and not a point key was flawed in that we'd miss ext points that were in between
the current and next sst keys, when we'd next both of them. This change addresses that.

It also addresses other miscellaneous corner cases around stats calculations with overlapping sst/engine range keys and point keys. All these bugs were found with the upcoming CheckSSTConflicts randomized test in #98408.

Epic: none

Release note: None

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Release justification: Low risk, high priority bugfix for an import / c2c-replication-related code path

[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 

[itsbilal]

TFTR!