client/network-gossip: Merge GossipEngine and GossipEngineInner (#5042)

* 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.

* client/finality-grandpa: Wrap GossipEngine in Arc Mutex & lock it on use

GossipEngine in itself has no need to be Send and Sync, given that it
does not rely on separately spawned background tasks anymore. Given that
finality-grandpa shares the `NetworkBridge` potentially between threads
its components need to be clonable, thus this patch wraps `GossipEngine`
in an `Arc<Mutex<>>`.