GRANDPA finality proof draft (#1268)

* grandpa finality proof

* prove GrandpaApi::grandpa_authorities using parent block + some docs

* create justification when consensus data is changed

* generate justifications periodically

* test for ConsensusChanges

substrate/node/cli/src/service.rs

@@ -100,6 +100,7 @@ construct_service_factory! {
 					grandpa::Config {
 						local_key,
 						gossip_duration: Duration::new(4, 0), // FIXME: make this available through chainspec?
+						justification_period: 4096,
 						name: Some(service.config.name.clone())
 					},
 					link_half,