Some fixes to BackupableDB

Summary:
(1) Report corruption if backup meta file has tailing data that was not read. This should fix: https://github.com/facebook/rocksdb/issues/81 (also, @sdong reported similar issue)
(2) Don't use OS buffer when copying file to backup directory. We don't need the file in cache since we won't be reading it twice
(3) Don't delete newer backups when somebody tries to backup the diverged DB (restore from older backup, add new data, try to backup). Rather, just fail the new backup.

Test Plan: backupable_db_test

Reviewers: ljin, dhruba, sdong

Reviewed By: ljin

CC: leveldb, sdong

Differential Revision: https://reviews.facebook.net/D16287