* 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>