eliminate a deadlock
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.