Avoid re-fragmenting memtable range tombstones on each read

Each time `MemTable::NewRangeTombstoneIterator` is called, the tombstones in the dedicated range deletion memtable are fragmented. Range deletions are probably issued as frequently as other forms of writes, so it should be possible to do some more work to avoid this in the average case. The fragmented tombstones can be cached, though I think doing more work on the write path is preferable, since we currently do the bare minimum of just writing a single entry to the memtable.