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<>>`.