Summary:
Backup engine is intentionally openable even when some backups are corrupt. Previously the engine could write new backups as long as the most recent backup wasn't corrupt. This PR makes the backup engine able to create new backups even when the most recent one is corrupt.

We now maintain two ID instance variables:

- `latest_backup_id_` is used when creating backup to choose the new ID
- `latest_valid_backup_id_` is used when restoring latest backup since we want most recent valid one
Closes https://github.com/facebook/rocksdb/pull/2804

Differential Revision: D5734148

Pulled By: ajkr

fbshipit-source-id: db440707b31df2c7b084188aa5f6368449e10bcf