dht find connected peers

This PR extends the DHT to send information re: peer "connectedness".
This means that --upon resolving dht queries -- connected peers will
signal whether or not they are directly connected to the peer. this is
in preparation for mesh messaging, and our "STUN" for NAT traversal.
#412 panics and takes down the daemon during `ipfs swarm connect`. i weep when i see more opportunities for panicking going into subsystems.