BEEFY: optimize voter event loop for fewer 'active' wakeups

[acatangiu]

Issue

Doing a bit of profiling showed BeefyWorker::run() async task being "active" more than it should, waking up many times even when there haven't been any external events relevant to the voter logic.

Investigating this showed the "culprit" to be listening for network-gossip events in the main async loop. Even if the "interesting" events are only NetEvent::SyncConnected and NetEvent::SyncDisconnected, the BEEFY voter actually wakes up and (uselessly) tries to vote for all network events (which are many!)

Proposed fix

Move network peers tracking to the much lighter BeefyJustifsRequestHandler::run() task (which is also the user of the connected peer set); thus massively decreasing wake-up rate of the heavier BEEFY voter task.

[acatangiu]

Actually GossipValidator has callbacks for peers connecting and disconnecting so just use that instead and avoid the extra async task.