SERVER-15913 QueryStageKeep dbtest shouldn't leak KeepMutationsStage
SERVER-15888: fix locking on collection mods in dbtests
SERVER-15888: fix lock for collection creation in profiling
SERVER-13635: fix rollback of collection create in kv engine
SERVER-15888: WriteContext will switch from MODE_IX to MODE_X if collection doesn't exist
SERVER-15888: add invariant check in kv engine that have correct lock for collection create/drop