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

[erikgrinaker]

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.

We should check whether the stats are accurate via ContainsEstimates before taking the point deletion path.