Add a new option for BackupEngine to store table files under shared_checksum using DB session id in the backup filenames (#6997)

Summary:
`BackupableDBOptions::new_naming_for_backup_files` is added. This option is false by default. When it is true, backup table filenames under directory shared_checksum are of the form `<file_number>_<crc32c>_<db_session_id>.sst`.

Note that when this option is true, it comes into effect only when both `share_files_with_checksum` and `share_table_files` are true.

Three new test cases are added.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6997

Test Plan: Passed make check.

Reviewed By: ajkr

Differential Revision: D22098895

Pulled By: gg814

fbshipit-source-id: a1d9145e7fe562d71cde7ac995e17cb24fd42e76