[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