Idea: Introduce "active only" nodes

[smashnet]

An "active only" node is a node that always actively establishes the connections to other nodes. This should be useful in case you cannot open a port in your firewall/router. This is especially the case if some of the peers use DualStack-Lite (IPv6 own address, IPv4 only via gateway) and others have real IPv4 addresses.

Of course this only works if few nodes are "active only".