client/beefy: ensure mandatory blocks on session changes by explicitly walking finality tree_route

[acatangiu]

Bug

Right now BEEFY voter assumes it's getting explicit finality notifications for any block containing authorities_change digest:

substrate/client/beefy/src/worker.rs

Lines 349 to 353 in 4d59804

	if let Some(new_validator_set) = find_authorities_change::<B>(header) {
	self.init_session_at(new_validator_set, *header.number());
	// TODO: when adding SYNC protocol, fire up a request for justification for this
	// mandatory block here.
	}

But zombienet tests in paritytech/polkadot#5840 show that is not always the case, sometimes the authorities_change block is in the middle of a sub-chain of blocks finalized at once.

Solution

Voter should drop above assumption and explicitly check if validator set changed, and if so, walk the tree_route in the finality notification to find and process the finalized block containing authorities_change digest.