[Compaction] Redesign RocksCompactionPicker to be the base class of the new level / universal compactions

Summary:
Redesign RocksCompactionPicker to be the base class of the new level / universal
compactions.  This redesign makes unifying level and universal compaction
more possible while giving the flexibility to level and universal compaction
to handle compaction differently in part.

Test Plan:
Include kRocksLevelCompaction and kRocksUniversalCompaction into the test
make db_test
./db_test
and all tests passed.

more tests will come up during the follow-up development.

Reviewers: sdong, ljin, igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20421