*: Register network event stream for authority discovery

Previously one would create a sender and receiver channel pair, pass the
sender to the `build_network_future` through the service builder and
funnel network events returned from polling the network service into the
sender to be consumed by the authority discovery module owning the
receiver.

With recent changes it is now possible to register an `event_stream`
with the network service directly, thus one does not need to make the
detour through the `build_network_future`.