rewrite of provides to better select peers to send RPCs to

The main change in this PR is the implementation of `getClosestPeers` that returns a channel of peers. progressively closer to the target key. This is used to put provider values to the best possible peers.
well, you get an arbitrary function: https://github.com/jbenet/go-ipfs/blob/master/routing/dht/routing.go#L299-L359