Fix tracking validator set in ImOnline (#3596)

* Use session::validators instead of staking::current_elected

* Basic test framework.

* Initialize validators, attempt to heartbeat.

* Use dummy crypto for im-online testing.

* Remove printlns.

* Finish test, make it invalid.

* Add reporting test.

* Finalize the test.

* Remove dumbness.

* Updates.

* Update AuRa

* Update srml/im-online/src/tests.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Derive Ord

* Add some more tests.

* Remove stray todo.

* Bump runtime version.

* Bump impl-trait-for-tuples.

* Enforce new version of trait-for-tuples.

substrate/srml/staking/src/lib.rs

@@ -269,7 +269,7 @@ use sr_primitives::traits::{
 };
 use phragmen::{elect, equalize, Support, SupportMap, ExtendedBalance, ACCURACY};
 use sr_staking_primitives::{
-	SessionIndex, CurrentElectedSet,
+	SessionIndex,
 	offence::{OnOffenceHandler, OffenceDetails, Offence, ReportOffence},
 };
 #[cfg(feature = "std")]
@@ -1586,12 +1586,3 @@ impl<T, Reporter, Offender, R, O> ReportOffence<Reporter, Offender, O>
 		}
 	}
 }
-
-/// Returns the currently elected validator set represented by their stash accounts.
-pub struct CurrentElectedStashAccounts<T>(rstd::marker::PhantomData<T>);
-
-impl<T: Trait> CurrentElectedSet<T::AccountId> for CurrentElectedStashAccounts<T> {
-	fn current_elected_set() -> Vec<T::AccountId> {
-		<Module<T>>::current_elected()
-	}
-}