Summary:
When a memtable is trimmed in MemTableListVersion, the memtable
is only added to delete list if it is
the last reference. However it is not the last reference as it is held
by the super version. But the super version would not be switched if the
delete list is empty. So the memtable is never destroyed and memory
usage increases beyond write_buffer_size +
max_write_buffer_size_to_maintain.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7296

Test Plan:
1.  ./db_bench -benchmarks=randomtransaction
-optimistic_transaction_db=1 -statistics -stats_interval_seconds=1
-duration=90 -num=500000 --max_write_buffer_size_to_maintain=16000000
--transaction_set_snapshot

Reviewed By: ltamasi

Differential Revision: D23267395

Pulled By: akankshamahajan15

fbshipit-source-id: 3a8d437fe9f4015f851ff84c0e29528aa946b650