Participant mode

[str4d]

It should be easy to configure Ire as a purely-participating router, i.e. a router that:

• Integrates itself into the network (via netDB exploration, possibly with a few exploratory tunnels).
• Collects peer profiles.
• Accepts tunnel build requests.
• Routes tunnel messages.

It might make sense to have this be a separate binary, that can be easily compiled and deployed (e.g. as a Docker image).

In order to be as lightweight as possible, this mode should probably not enable floodfill mode (#46).