Stop udp-sender processes when canceling a session.

[Sebastian-Roth]

Speaking of Multicast, I've noticed that if the sessions don't start at all, but still when you cancel all Multicast tasks in the web panel, the udp-sender itself remains running in the background on the server.

Ideally, after cancelling all tasks, the udp-sender should be killed in any case.

Originally posted by @Piotr86PL in FOGProject/fos#48 (comment)