[Compaction] Code cleanup for preparing merging compaction branch into master.

Summary:
Remove / move part of compaction related code in the include/rocksdb
directory that is not currently used by the CompactFiles API (but will
be used in the later compaction refactory) to prepare the first phase
merging compaction branch into master.

Test Plan: db_test and compactor_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22587
[Compaction] Cleanup II
[Compaction] Move CompactionJob from db.h to db_impl.h

Summary:
Move CompactionJob from db.h to db_impl.h as it's not currently
used by CompactFiles API, but will be moved to db.h once again
when PluggableCompactionPicker is ready.

Test Plan: db_test and compactor_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22593
[Compaction] Renamed an internal function to improve readability.