This implements a more standard API for instantiating a homeserver and
moves some of the dependency injection into the test suite.
More concretely this stops using `setattr` on all `kwargs` passed to `HomeServer`.
This ended up being a bit more invasive than I'd hoped for (not helped by
generic_worker duplicating some of the code from homeserver), but hopefully
it's an improvement.
The idea is that, rather than storing unstructured `dict`s in the config for
the listener configurations, we instead parse it into a structured
`ListenerConfig` object.
