Summary: Default crash test uses prefix hash memtable, which is not compatible to concurrent memtable. Allow prefix test run with skip list and use skip list memtable when concurrent insert is used.
Test Plan: Run "python -u tools/db_crashtest.py whitebox" and watch sometimes skip list is used.
Reviewers: anthony, yhchiang, kradhakrishnan, andrewkr, IslamAbdelRahman
Reviewed By: IslamAbdelRahman
Subscribers: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D53907
2608219cc9