p2p: Add timeouts to signaling discovery/exchange states

[iostat]

State	Channel	Waiting for
DiscoveryRequested	Discovery	Discovered response
Discovered	Discovery	Dialer's accept/reject
DiscoveredAccepted	Discovery	Offer encryption and send
DiscoveredAccepted (remote)	Discovery	Answer from listener
Offered	Exchange	Answer from listener

If a relay peer goes silent, becomes unresponsive, or the listener never
answers, the requesting side waits indefinitely. Resources are consumed with
no recovery path.