netlink-proto: if messages are never acknowledged, the Protocol.pending_requests keep growing

[little-dude]

There should be an option to set a timer to remove pending requests.

[stbuehler]

I'd prefer a solution that unregisters requests when the receiver is dropped. Both solutions could be resulting in old messages turning up in newer response channels, but timers seem way more complicated and dangerous to me. Maybe there is a way to slow down requests when the sequence id gets cycled too fast?

And #209 should probably get fixed before, because removing requests means there is a higher chance for unsolicited messages.