Fix double deletes

Summary: While debugging clients compaction issues, I noticed bunch of delete bugs: P16329995. MakeTableName returns sst file with "/" prefix. We also need "/" prefix when we get the files though GetChildren(), so that we can properly dedup the files.

Test Plan: none

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23457
CompactedDBImpl

Summary:
Add a CompactedDBImpl that will enabled when calling OpenForReadOnly()
and the DB only has one level (>0) of files. As a performan comparison,
CuckooTable performs 2.1M/s with CompactedDBImpl vs. 1.78M/s with
ReadOnlyDBImpl.

Test Plan: db_bench

Reviewers: yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23553