Summary: If users turn on concurrent insert but the memtable doesn't support it, they might see unexcepted crash. Fix it by explicitly fail.

Test Plan:
Run different setting of stress_test and make sure it fails correctly.
Will add a unit test too.

Reviewers: anthony, kradhakrishnan, IslamAbdelRahman, yhchiang, andrewkr, ngbronson

Reviewed By: ngbronson

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D53895