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.
> but it's really not a big deal. i don't forsee a bug being introduced due to the decision.

agreed. same here, i think this was a vestige from before those funcs. addressed in 93872a5

> Looking at the big picture, where would you want me to focus CR attention? Is there anything particular you're concerned about?

Mostly the notion of "connectedness" and DHT logic. the added dht query https://github.com/jbenet/go-ipfs/pull/428/files#diff-b4153ad02ca247a808e7a06fd1e2f133R272 -- it's mostly the same as the others, with small tweaks