Summary: Backup options file to private directory
Test Plan:
backupable_db_test.cc, BackupOptions
Modify DB options by calling OpenDB for 3 times. Check the latest options file is in the right place. Also check no redundent files are backuped.
Reviewers: andrewkr
Reviewed By: andrewkr
Subscribers: leveldb, dhruba, andrewkr
Differential Revision: https://reviews.facebook.net/D59373