Summary: added a new option to BackupEngine: if share_files_with_checksum is set to true, sst files are stored in shared_checksum/ and are identified by the triple (file name, checksum, file size) instead of just the file name. This option is targeted at distributed databases that want to backup their primary replica.

Test Plan: unit tests and tested backup and restore on a distributed rocksdb

Reviewers: igor

Reviewed By: igor

Differential Revision: https://reviews.facebook.net/D18393