storage,kv: calculate approximate MVCC stats during compactions

Today, write operations calculate exact deltas to MVCC stats, requiring always reading before writing. This is an obstacle to providing availability during transient failure of physical storage (cockroachdb/pebble#3230). It also reduces write-path performance. The need to maintain exact stats across MVCC GC is also an obstacle to compaction-time MVCC garbage collection. cockroachdb/cockroach#57260.

Alternatively, we could estimate MVCC stats during compactions and rely on those estimates.

TODO: complete

https://docs.google.com/document/d/1fCRO3UJjQ-_nfyfPp0pdMvATqJ4oT1LnZYhPy92DPZg/edit#heading=h.n38grdlapoea

Jira issue: CRDB-37054