Summary: we should not split file on level 0 in compaction because it will fail the following verification of seqno order on level 0

Test Plan: check with filldeterministic in db_bench

Reviewers: yhchiang, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D65193