Summary:
This diff fixes a bug introduced by D28521. Read-only backup engine can delete a backup that is later than the latest -- we never check the condition.
I also added a bunch of logging that will help with debugging cases like this in the future.
See more discussion at t6218248.
Test Plan: Added a unit test that was failing before the change. Also, see new LOG file contents: https://phabricator.fb.com/P19738984
Reviewers: benj, sanketh, sumeet, yhchiang, rven, sdong
Reviewed By: sdong
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D33897
b9ff6b050d