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/staking/src/lib.rs

@@ -1355,6 +1355,10 @@ impl<T: Trait> Module<T> {
 			// Set the new validator set in sessions.
 			<CurrentElected<T>>::put(&elected_stashes);
 
+			// In order to keep the property required by `n_session_ending`
+			// that we must return the new validator set even if it's the same as the old,
+			// as long as any underlying economic conditions have changed, we don't attempt
+			// to do any optimization where we compare against the prior set.
 			(slot_stake, Some(elected_stashes))
 		} else {
 			// There were not enough candidates for even our minimal level of functionality.