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