server/keyexch.c: Randomized exponential retransmit backoff.

Rather than retrying after a constant delay, increase the delay by a
constant factor.  We now start with a smaller retransmit interval, and
cap at a fairly long wait.  Also introduce a random `wobble'
proportional to the delay in order to prevent a server being hammered by
a swarm of clients acting in unison if it comes back after an outage.