GRANDPA links set IDs to sessions. (#3472)

* introduce some type aliases for round and set-id

* overhaul session "changed" flag and document better

* do_initialize in BABE when getting new session

* grandpa module tracks set IDs

* update runtime versions

* doc comment -> comment

* Include docs fixes from Gav

Co-Authored-By: Gavin Wood <gavin@parity.io>

* some more review changes

* fix srml-grandpa compilation

substrate/srml/babe/src/lib.rs

@@ -445,6 +445,8 @@ impl<T: Trait> session::OneSessionHandler<T::AccountId> for Module<T> {
 	fn on_new_session<'a, I: 'a>(_changed: bool, validators: I, queued_validators: I)
 		where I: Iterator<Item=(&'a T::AccountId, AuthorityId)>
 	{
+		Self::do_initialize();
+
 		// Update epoch index
 		let epoch_index = EpochIndex::get()
 			.checked_add(1)