Free obsolete memtables outside the dbmutex had a memory leak.

Summary:
The commit at 27bbef11802d27c80df7e0b27091876df23b9986 had a memory leak
that was detected by valgrind. The memtable that has a refcount decrement
in MemTableList::InstallMemtableFlushResults was not freed.

Test Plan: valgrind ./db_test --leak-check=full

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14391