Summary:
Even if hard limit hits, flushing more memtable may not help cap the memory usage if already more than half data is scheduled for flush. Not triggering flush instead.
Closes https://github.com/facebook/rocksdb/pull/2469

Differential Revision: D5284249

Pulled By: siying

fbshipit-source-id: 8ab7ba1aba56a634dbe72b318fcab2093063972e