Limit (incoming) connecting peers.

[kdeme]

Currently in our implementation, the maximum connected peers limit is only applied on the creation of new outgoing connections.

The listener for incoming connections remains active. This gives a serious risk and should be handled by either:

1. Being the very friendly peer: allowing the handshake to start and then send a disconnect message with ToManyPeers as DisconnectionReason
2. Ignore any incoming connections and just disconnect them without any message.
3. Don't allow the incoming connection at all?

I'm guessing when going from 1. down, the possibility of being seen as a "bad" peer will go up. However, practically, I'm not sure if any client actually cares.

[arnetheduck]

One thing to consider is that incoming peers should probably be counted separately - there's a well-known attack where you flood a peer with your own connections by connecting to them and eventually eclipsing them - to mitigate this, we want a minimum number of outgoing connections to peers of our own choosing.

[kdeme]

Yeah, good point.
I had the same concern and created #111 because of that. But counting separately might just be enough to mitigate that.