DB to only flush the column family with the largest memtable while option.db_write_buffer_size is hit

Summary: When option.db_write_buffer_size is hit, we currently flush all column families. Move to flush the column family with the largest active memt table instead. In this way, we can avoid too many small files in some cases.

Test Plan: Modify test DBTest.SharedWriteBuffer to work with the updated behavior

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

Reviewed By: igor

Subscribers: march, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D51291