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/executor/src/lib.rs

@@ -294,7 +294,6 @@ mod tests {
 	}
 
 	fn new_test_ext(code: &[u8], support_changes_trie: bool) -> TestExternalities<Blake2Hasher> {
-		let three = AccountId::from_raw([3u8; 32]);
 		let mut ext = TestExternalities::new_with_code(code, GenesisConfig {
 			aura: Some(Default::default()),
 			system: Some(SystemConfig {
@@ -319,7 +318,6 @@ mod tests {
 				vesting: vec![],
 			}),
 			session: Some(SessionConfig {
-				validators: vec![AccountKeyring::One.into(), AccountKeyring::Two.into(), three],
 				keys: vec![
 					(alice(), to_session_keys(&AuthorityKeyring::Alice)),
 					(bob(), to_session_keys(&AuthorityKeyring::Bob)),