Default connection handlers should be an object, not a dict

[itamarst]

Right now, the configuration for outgoing TCP connections, e.g. "TCP should use Tor" is a dictionary, since that is what Foolscap requires.

Given a move away from Foolscap, there are other places using this information, e.g. the HTTP storage protocol. In those situations one might want to ask "is this using Tor?", and a dictionary of strings is not a very well-structured way of getting this information.