range_tombstone_accumulator: Avoid update_current_tombstone() when nothing changed
range_tombstone_accumulator: Avoid update_current_tombstone() when nothing changed

Recalculation of the current tombstone is O(N) in the number of active
range tombstones. This can be a significant overhead, so better avoid it.

Solves the problem of quadratic complexity when producing lots of
overlaping range tombstones with a common end bound.

Refs #8625
Refs #8626

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla