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>