Read-only BackupEngine

Summary: Read-only BackupEngine can connect to the same backup directory that is already running BackupEngine. That enables some interesting use-cases (i.e. restoring replica from primary's backup directory)

Test Plan: added a unit test

Reviewers: dhruba, haobo, ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18297