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/node/runtime/src/lib.rs

@@ -71,8 +71,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 103,
-	impl_version: 103,
+	spec_version: 104,
+	impl_version: 104,
 	apis: RUNTIME_API_VERSIONS,
 };
 
@@ -197,6 +197,7 @@ impl session::Trait for Runtime {
 	type ShouldEndSession = session::PeriodicSessions<Period, Offset>;
 	type Event = Event;
 	type Keys = SessionKeys;
+	type SelectInitialValidators = Staking;
 }
 
 parameter_types! {