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