MedusaBackupJob status updates are delayed until completion

[adejanovski]

It looks like we can't track the progress of backup operations in the MedusaBackupJob status as it only seems to update at the very end of the backup.
Only the startTime is filled in, and then until all nodes are backed up, nothing happens.
After the backup has completed on all nodes, we see the status being updated with the list of nodes that finished the backup.

Another thing to note is that nodes seem to be backed up sequentially instead of concurrently. #1231.