Improve move cancellation

[rwb27]

Currently, moves >5000 steps are done non-blocking. This is slightly inelegant, because it involves quite a few queries:

• Disable blocking moves
• Start the move
• LOOP
  • Query if we're moving
  • Wait
• Re-enable blocking moves

There's also jitter that will come from the Python code waiting in between polls.

A much more elegant way would be to add a timeout to wait_until_stopped (would require firmware update) so that we can long-poll it. I'd be pretty confident that this wouldn't affect the timings noticeably, so we could then make moves non-blocking by default.