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.
Or instead of allowing a count to be specified, use `KValue` hardcoded, to match the kademlia spec. (the only value ever passed into count is `KValue`)