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.
may not need it, given the functions: https://github.com/jbenet/go-ipfs/pull/428/files#diff-d7b3509b9fcd5fce42069cf169ede81eR142

do you care much either way?