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/core/primitives/src/offchain.rs

@@ -160,7 +160,7 @@ pub struct OpaqueNetworkState {
 }
 
 /// Simple blob to hold a `PeerId` without committing to its format.
-#[derive(Clone, Eq, PartialEq, Encode, Decode)]
+#[derive(Default, Clone, Eq, PartialEq, Encode, Decode)]
 #[cfg_attr(feature = "std", derive(Debug))]
 pub struct OpaquePeerId(pub Vec<u8>);