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