Implement retry logic around starting exclusive backup operation

Specifically, the client does a checkAndPut to specifics coordinates in the backup table and throws an exception when that fails. Remember that backups are client driven (per some design review from a long time ago), so queuing is tough to reason about (we have no "centralized" execution system to use). At a glance, it seems pretty straightforward to add some retry/backoff semantics to BackupSystemTable#startBackupExclusiveOperation().

While we are in a state in which backup operations cannot be executed in parallel, it would be nice to provide some retry logic + configuration. This would alleviate users from having to build this themselves.