Fix DBCompactionTestWithParam.CompactionTrigger in non-jemalloc build.

Fix DBCompactionTestWithParam.CompactionTrigger in non-jemalloc build.

Summary: DBCompactionTestWithParam.CompactionTrigger fails in non-jemalloc build, after the skip list memtable change. Fix it by making mem table flush trigger by number of entries.

Test Plan: Run the test using both of jemalloc and non-jemalloc build.

Reviewers: anthony, IslamAbdelRahman, rven, kradhakrishnan, igor, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D51471