staking: only disable slashed validators and keep them disabled for whole era (#9448)

* session: remove disabled validators threshold logic

* staking: add logic to track offending validators

* staking: disable validators for the whole era

* frame: fix tests

* staking: add tests for disabling validators handling

* staking: fix adding offending validator when already slashed in era

* address review comments

* session, staking: add comments about sorted vecs

Co-authored-by: Andronik Ordian <write@reusable.software>

substrate/frame/beefy/src/lib.rs

@@ -162,7 +162,7 @@ impl<T: Config> OneSessionHandler<T::AccountId> for Pallet<T> {
 		}
 	}
 
-	fn on_disabled(i: usize) {
+	fn on_disabled(i: u32) {
 		let log: DigestItem<T::Hash> = DigestItem::Consensus(
 			BEEFY_ENGINE_ID,
 			ConsensusLog::<T::BeefyId>::OnDisabled(i as AuthorityIndex).encode(),

substrate/frame/beefy/src/mock.rs

@@ -105,7 +105,6 @@ impl pallet_session::Config for Test {
 	type SessionManager = MockSessionManager;
 	type SessionHandler = <MockSessionKeys as OpaqueKeys>::KeyTypeIdProviders;
 	type Keys = MockSessionKeys;
-	type DisabledValidatorsThreshold = DisabledValidatorsThreshold;
 	type WeightInfo = ();
 }