Enforce disk-reader to be after the disk-writer

Enforce disk-reader to be after the disk-writer

If disk-monitoring is disabled: disk-reader position is not relevant.
If Rec-arm is off: disk-writer position is not relevant.

But...

   Play -> [plugins] -> Record

is basically a bounce and best done using the bounce operation.
(faster than realtime).

   Input + Play -> Record -> Output

Ardour would need to align playback with the Input to be recorded
and at the same time align it with output, so that a player can play
along on the same track. That's not possible without a time-machine (or
a 2nd play processor).

While it can work in theory under some special circumstances, allowing
the disk-reader before the disk-writer is really just confusing,
error prone and valid uses cases are better handled by dedicated
operations.