Summary:
the range delete tombstones in memtable should be added to the aggregator even when the memtable's prefix bloom filter tells us the lookup key's not there. This bug could cause data to temporarily reappear until the memtable containing range deletions is flushed.

Reported in #2743.
Closes https://github.com/facebook/rocksdb/pull/2745

Differential Revision: D5639007

Pulled By: ajkr

fbshipit-source-id: 04fc6facb6f978340a3f639536f4ca7c0d73dfc9