* client/network-gossip/src/bridge: Finish when network even stream closes

Previously within `<GossipEngine as Future>::poll` one would poll the
`network_event_stream` ignoring all messages other than
`Poll::Ready(Some())`. Ignoring `Poll::Ready(None)` leads to a panic on
the next poll of the stream, gien that it is not fused.

By design `network_event_stream` does not close unless an unbounded send
into it fails, or the `NetworkWorker` gets shut down.

> The stream never ends (unless the `NetworkWorker` gets shut down).
> (client/network/src/service.rs)

An `unbounded_send` to fail on an unbounded channel is unlikely. The
`NetworkWorker` shutting down is not unlikely. In such case the
`GossipEngine` should shut down as well.

With this patch a `<GossipEngine as Future>` finishes on
`Poll::Ready(None)` returned from `network_event_stream`.

* client/finality-grandpa/communication: Error on gossip engine finished

Have `<NetworkBridge as Future>::poll` return `Poll::Ready(Err)` instead
of `Poll::Ready(Ok)` to be consistent with the handling of the neighbor
packet worker stream and the gossip validator report stream. Both `Err`
as well as `Ok` shut down the `NetworkBridge` as well as the
`VoterWorker`.

* client/network-gossip/src/bridge: Add regression test

* client/network-gossip: Move substrate test client to dev dependencies

* client/network-gossip: Remove TODO

Addressed in a follow up pull request.

* client/network-gossip/bridge: Put match on newline after loop

* client/finality-grandpa/src/observer: Fix regression test

Make sure the event stream sender side is not dropped till the end.