dht: helpful debugging for no closer peers
bitswap: network interface changed

Had to change the network interface from DialPeer(peer.ID) to
DialPeer(peer.PeerInfo), so that addresses of a provider are
handed to the network.

@maybebtc and I are discussing whether this should go all the
way down to the network, or whether the network _should always
work_ with just an ID (which means the network needs to be
able to resolve ID -> Addresses, using the routing system.
This latter point might mean that "routing" might need to
break down into subcomponents. It's a bit sketchy that
the Network would become smarter than just dial/listen and
I/O, but maybe there's a distinction between net.Network,
and something like a peernet.Network that has routing
built in...)