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/session/src/mock.rs

@@ -29,7 +29,6 @@ use sp_runtime::{
 	impl_opaque_keys,
 	testing::{Header, UintAuthorityId},
 	traits::{BlakeTwo256, ConvertInto, IdentityLookup},
-	Perbill,
 };
 use sp_staking::SessionIndex;
 
@@ -144,7 +143,7 @@ impl SessionHandler<u64> for TestSessionHandler {
 				.collect()
 		});
 	}
-	fn on_disabled(_validator_index: usize) {
+	fn on_disabled(_validator_index: u32) {
 		DISABLED.with(|l| *l.borrow_mut() = true)
 	}
 	fn on_before_session_ending() {
@@ -269,10 +268,6 @@ impl pallet_timestamp::Config for Test {
 	type WeightInfo = ();
 }
 
-parameter_types! {
-	pub const DisabledValidatorsThreshold: Perbill = Perbill::from_percent(33);
-}
-
 impl Config for Test {
 	type ShouldEndSession = TestShouldEndSession;
 	#[cfg(feature = "historical")]
@@ -284,7 +279,6 @@ impl Config for Test {
 	type ValidatorIdOf = ConvertInto;
 	type Keys = MockSessionKeys;
 	type Event = Event;
-	type DisabledValidatorsThreshold = DisabledValidatorsThreshold;
 	type NextSessionRotation = ();
 	type WeightInfo = ();
 }