smarter udp-control scheduling

[Antoine Martin]

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.