server/peer.c, server/tests.at: Handle NAT swapping over peer addresses.

Previously, we'd notice when a mobile peer switched address (e.g., due
to NAT) to one that wasn't currently known; but it seems that some
particularly awful NAT boxes will actually swap over the addresses
assigned to two peers on the wrong side of it.  The static hub peer will
see this as a bunch of decryption failures but not do anything about
it.  Eventually the mobile peers will notice a ping timeout and
reconnect, but this can take a while.

So, when we get a decryption failure from a mobile peer, see whether any
other mobile peers are interested in it.