client/network-gossip: Merge GossipEngine and GossipEngineInner

Given that GossipEngine and GossipEngineInner are not shared between
threads anyone (public interface + background tasks), neither depends on
being Send or Sync. Thus one can merge the two as done in this patch.
One only needs to wrap an `Arc<Mutex<>>` around the whole structure when
the owner (e.g. finality-grandpa) needs to share the gossip engine
between threads.