Session keys buffered for a session. (#2946)

* Session keys buffered for the duration of a session.

* Add queued_keys getter.

* Make sure genesis state is consistent.

* Add validator_count validators.

* Compensate for session delay.

* Remove unused code.

* Add num_validators option.

* Fix session numbers.

* Fix merge.

* Reintroduce changed.

* Update runtime.

* Make NextKeyFor private.

* Move block initialization to function.

* Update lib.rs

* Add test for change propagation.

* Fix docstring.

* Use get instead of take.

* Initialize validators from keys.

* Next try.

* Fix build.

* Fix warning.

* Make initial validator selection more transparent.

* Make storage items private.

* Reorder genesis initialization.

* Update Cargo.lock

* Update runtime version.

* Update runtime version.

* Update Cargo.lock

* Update runtime version.

* Add docs.

substrate/srml/staking/src/lib.rs

@@ -281,7 +281,7 @@ use srml_support::{
 		WithdrawReasons, OnUnbalanced, Imbalance, Get
 	}
 };
-use session::{OnSessionEnding, SessionIndex};
+use session::{OnSessionEnding, SelectInitialValidators, SessionIndex};
 use primitives::Perbill;
 use primitives::traits::{
 	Convert, Zero, One, StaticLookup, CheckedSub, CheckedShl, Saturating, Bounded
@@ -587,10 +587,6 @@ decl_storage! {
 						}, _ => Ok(())
 					};
 				}
-
-				if let (_, Some(validators)) = <Module<T>>::select_validators() {
-					<session::Validators<T>>::put(&validators);
-				}
 			});
 		});
 	}
@@ -1255,3 +1251,9 @@ impl<T: Trait> OnFreeBalanceZero<T::AccountId> for Module<T> {
 		<Nominators<T>>::remove(stash);
 	}
 }
+
+impl<T: Trait> SelectInitialValidators<T> for Module<T> {
+	fn select_initial_validators() -> Option<Vec<T::AccountId>> {
+		<Module<T>>::select_validators().1
+	}
+}