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`.