Optional checks for ongoing balance and scrub

I think it would be useful with some checks so that btrbk doesn't do snapshots or transfers while either device is running scrub or balance type actions. According to #btrfs this 'could' perhaps cause unexpected behavior. At the very least it would be very slow.

No files, tests, or entry points are named in the issue. Start by locating the code that initiates snapshots and transfers, then identify how btrfs scrub and balance activity can be detected; done means optional checks reliably prevent those operations while either activity is running.

Written by the indexing model from the issue text.