Continuation of #70

After a bit of prototyping, my plan here is to make a sorta-breaking-but-coincidentally-compatible change to UpdateResponse such that ActivePeers are actually enode:// strings rather than node IDs. The agents don't care what those values are right now (so it's technically backwards compatible), so this would effectively just change the pool implementation and the new StrictPeers feature will use it.

For consistency, I'd also like to change the InactivePeers to be the same, but that would break the existing agents, so I'm going to change the code to more defensively extract the node IDs out of InactivePeers in the next release, and then change the pool implementation in the future.