Pallet session new API (#4609)

* Initial work

* Fix most things

* fix test

* fix old comment

* migration

* fix

* remove useless stuff

* fix

* less spaghetti implementation

* fix initial session

* fix

substrate/frame/babe/src/mock.rs

@@ -79,9 +79,8 @@ impl pallet_session::Trait for Test {
 	type ValidatorId = <Self as frame_system::Trait>::AccountId;
 	type ShouldEndSession = Babe;
 	type SessionHandler = (Babe,Babe,);
-	type OnSessionEnding = ();
+	type SessionManager = ();
 	type ValidatorIdOf = ();
-	type SelectInitialValidators = ();
 	type Keys = MockSessionKeys;
 	type DisabledValidatorsThreshold = DisabledValidatorsThreshold;
 }