Summary: Seems like the cleanest way to resolve this is to move CompactedDBImpl into db/. CompactedDBImpl should probably live in the same place as DBImplReadonly since Opening the latter could end up instantiating the former. Both DBImplReadonly and CompactedDBImpl inherit from DBImpl access protected members of DBImpl( and the latter access friendly private methods).
Test Plan: unit tests
Reviewers: sdong, yhchiang, kradhakrishnan, igor
Reviewed By: igor
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D42027
