Export BackupEngine

Summary:
Lots of clients have problems with using StackableDB interface. It's nice to have BackupableDB as a layer on top of DB, but not necessary.

This diff exports BackupEngine, which can be used to create backups without forcing clients to use StackableDB interface.

Test Plan: backupable_db_test

Reviewers: dhruba, ljin, swk

Reviewed By: ljin

CC: leveldb, benj

Differential Revision: https://reviews.facebook.net/D15477