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/im-online/src/lib.rs

@@ -919,7 +919,7 @@ impl<T: Config> OneSessionHandler<T::AccountId> for Pallet<T> {
 		}
 	}
 
-	fn on_disabled(_i: usize) {
+	fn on_disabled(_i: u32) {
 		// ignore
 	}
 }

substrate/frame/im-online/src/mock.rs

@@ -27,7 +27,7 @@ use sp_core::H256;
 use sp_runtime::{
 	testing::{Header, TestXt, UintAuthorityId},
 	traits::{BlakeTwo256, ConvertInto, IdentityLookup},
-	Perbill, Permill,
+	Permill,
 };
 use sp_staking::{
 	offence::{OffenceError, ReportOffence},
@@ -146,10 +146,6 @@ parameter_types! {
 	pub const Offset: u64 = 0;
 }
 
-parameter_types! {
-	pub const DisabledValidatorsThreshold: Perbill = Perbill::from_percent(33);
-}
-
 impl pallet_session::Config for Runtime {
 	type ShouldEndSession = pallet_session::PeriodicSessions<Period, Offset>;
 	type SessionManager =
@@ -159,7 +155,6 @@ impl pallet_session::Config for Runtime {
 	type ValidatorIdOf = ConvertInto;
 	type Keys = UintAuthorityId;
 	type Event = Event;
-	type DisabledValidatorsThreshold = DisabledValidatorsThreshold;
 	type NextSessionRotation = pallet_session::PeriodicSessions<Period, Offset>;
 	type WeightInfo = ();
 }