[Compaction] Make RocksCompactionPickerUniversal use level structure.

Summary:
RocksCompactionPickerUniversal now implements universal compaction
using level structure.  The new universal compaction is now using
the same code path as level compaction.

TODO: deal with manual compaction

Test Plan: passes all db_test except those involves in manual compaction

Reviewers: sdong, ljin, igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20469