service/src/lib.rs: Register network event stream for authority disc

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.

This commit is an adjusted clone of one targeting the Substrate
repository.