Reset UTCP timers after a succesful key exchange.

The problem is that MeshLink does key exchange with peers on-demand. One
has to start a channel for key exchange to be initiated, but it also tries
to send the first SYN packet immediately. This will not work, and then it
has to wait for the retransmit timer to expire before it will try to send
the SYN again. Since the retransmit timer is increased exponentially, this
can potentially take a long time. So when key exchange is finished, we now
force a reset of the timers of all UTCP connections with the peer.