grandpa: avoid importing unnecessary justifications (#14423)

* grandpa: avoid importing unnecessary justifications

* grandpa: make justification_import_period configurable

* grandpa: keep the first justification

* grandpa: add test for justification import period

* grandpa: fix test

substrate/bin/node-template/node/src/service.rs

@@ -37,6 +37,10 @@ pub(crate) type FullClient =
 type FullBackend = sc_service::TFullBackend<Block>;
 type FullSelectChain = sc_consensus::LongestChain<FullBackend, Block>;
 
+/// The minimum period of blocks on which justifications will be
+/// imported and generated.
+const GRANDPA_JUSTIFICATION_PERIOD: u32 = 512;
+
 #[allow(clippy::type_complexity)]
 pub fn new_partial(
 	config: &Configuration,
@@ -97,6 +101,7 @@ pub fn new_partial(
 
 	let (grandpa_block_import, grandpa_link) = sc_consensus_grandpa::block_import(
 		client.clone(),
+		GRANDPA_JUSTIFICATION_PERIOD,
 		&client,
 		select_chain.clone(),
 		telemetry.as_ref().map(|x| x.handle()),
@@ -290,7 +295,7 @@ pub fn new_full(config: Configuration) -> Result<TaskManager, ServiceError> {
 		let grandpa_config = sc_consensus_grandpa::Config {
 			// FIXME #1578 make this available through chainspec
 			gossip_duration: Duration::from_millis(333),
-			justification_period: 512,
+			justification_generation_period: GRANDPA_JUSTIFICATION_PERIOD,
 			name: Some(name),
 			observer_enabled: false,
 			keystore,