Companion PR for Substrate#4394 (#723)

* 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.

* service/src/lib.rs: Fix futures::stream imports

* [TMP] *: Replace polkadot-upstream with feature branch

* Switch branch

* Small change

* Companion PR to substrate#4542

* Revert "Merge remote-tracking branch 'tomaka/companion-4542' into ashley-browser-utils"

This reverts commit 17f00afe, reversing
changes made to 928cbb9c

.

* ashley-browser-utils -> ashley-browser-utils-polkadot

* Switch branches back

Co-authored-by: Max Inden <[email protected]>
Co-authored-by: Pierre Krieger <[email protected]>