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/core/finality-grandpa/primitives/src/lib.rs

@@ -52,6 +52,12 @@ pub type AuthorityWeight = u64;
 /// The index of an authority.
 pub type AuthorityIndex = u64;
 
+/// The identifier of a GRANDPA set.
+pub type SetId = u64;
+
+/// The round indicator.
+pub type RoundNumber = u64;
+
 /// A scheduled change of authority set.
 #[cfg_attr(feature = "std", derive(Debug, Serialize))]
 #[derive(Clone, Eq, PartialEq, Encode, Decode)]