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/node/runtime/src/lib.rs

@@ -83,7 +83,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 157,
-	impl_version: 158,
+	impl_version: 159,
 	apis: RUNTIME_API_VERSIONS,
 };
 
@@ -409,7 +409,6 @@ impl im_online::Trait for Runtime {
 	type Event = Event;
 	type SubmitTransaction = SubmitTransaction;
 	type ReportUnresponsiveness = Offences;
-	type CurrentElectedSet = staking::CurrentElectedStashAccounts<Runtime>;
 }
 
 impl offences::Trait for Runtime {