Following up #2554.
Instead of calling
schedule_control with just a delay, add a callback to check if sending the 'udp-control' packet is still needed.
With out of order packets, we can just check if the missing packet has actually been received before sending.
Difficulty: need to keep all the delays and callbacks around so that if one is discarded we can check the next one.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2555