Allow possibly avoiding overlap checks in the case where the source of
the min-live timestamp is known to only contain data which was written
*after* expiry treshold. Expiry treshold is the upper bound of
tombstone.deletion_time that was already expired at the time of
obtaining this expiry treshold value. Meaning that any write originating
from after this point in time, was generated at a time when such
tombstone was already expired. Hence these writes are not relevant for
the purposes of overlap checks with the tombstone and so their min-live
timestamp can be ignored.
This is important for MV workloads, where writes generated now can have
timestamps going far back in time, possibly blocking tombstone GC of
much older [shadowable] tombstones.
cfac9691ff