Add a "Disk Only" flag for upr streams

This flag will allow us to instantiate a stream that only reads off of disk and once finished it will end the stream. This behavior is needed by the backup client in order to allow the backup to get a single snapshot.

If you take a look at the upr-stream.cc/.h files you will see that there is a state machine for how a stream retrieves items to be sent. Once the backfill phase is finished we should check to see if this flag exists. If it does we should send a close stream message and transition the stream to dead state.