This was rather impenetrable and had some significant code
duplication. So add some words of explanation, and refactor to remove
the duplication.
Some fly-by fixing of minor bug too: buildUnicastRoutes failed to
exclude asymmetric connections from the first hop.
Golang locks aren't re-entrant. As result one can get deadlocks on
RWMutexes like this:
goroutine #1: attempt to acquire read lock -> success
goroutine #2: attempt to acquire write lock -> wait
goroutine #1: attempt to acquire read lock again -> wait
(a pending write lock prevents subsequent acquisitions of read locks)
Peer.HasPathTo could run into this since it could attempt to acquire
the read lock on a peer multiple times if previously it had
encountered that peer via an asymmetric connection.
rade