Avoid iterating the memtable contents when doing range queries with `only_purge_repaired_tombstones` enabled

Currently we iterate over the subrange of the memtable contents we are querying for to find the oldest unrepaired tombstone, we could probably just track the oldest tombstone globally for the memtable without any big drawbacks (we might include a few more tombstones when doing the read).

Also noted that we use EncodingStats#minLocalDeletionTime without checking for the DELETION_TIME_EPOCH value in a few places, this can make us include too many expired tombstones during reads.