Limit number of incoming connections (#391)

* Limit number of incoming connections

* Check Endpoint::Listener before checking num_open_connections.incoming

* Maintain at least 1-1/n portion of outgoing connections

* Remove use

* Default incoming_peers_factor to 2

* Use max_incoming_peers and max_outgoing peers to check whether connections should be dropped

* Fix expected_max_peers: reserved peers are not counted in config.max_peers

* typo: fix test