Peering tweaks

`sharePeers` works a bit differently now. It still sends up to 10 random peers, but there shouldn't be any duplicates anymore.

There is also some basic peer culling now. It's pretty unforgiving: if a peer does not respond to a broadcast, it will immediately be removed. Down the road I'll implement something more sophisticated, like sorting the peer list by last message time (I think Bitcoin does this).