BEEFY: optimize voter event loop for fewer 'active' wakeups (#12760)

* client/beefy: remove high-freq network events from main loop

Network events are many and very frequent, remove the net-event-stream
from the main voter loop and drastically reduce BEEFY voter task
'wakeups'.

Instead have the `GossipValidator` track known peers as it already
has callbacks for that coming from `GossipEngine`.

Signed-off-by: acatangiu <adrian@parity.io>

substrate/client/beefy/src/communication/gossip.rs

@@ -139,6 +139,10 @@ impl<B> Validator<B> for GossipValidator<B>
 where
 	B: Block,
 {
+	fn peer_disconnected(&self, _context: &mut dyn ValidatorContext<B>, who: &PeerId) {
+		self.known_peers.lock().remove(who);
+	}
+
 	fn validate(
 		&self,
 		_context: &mut dyn ValidatorContext<B>,