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/cli/src/service.rs

@@ -54,6 +54,10 @@ type FullGrandpaBlockImport =
 /// The transaction pool type defintion.
 pub type TransactionPool = sc_transaction_pool::FullPool<Block, FullClient>;
 
+/// The minimum period of blocks on which justifications will be
+/// imported and generated.
+const GRANDPA_JUSTIFICATION_PERIOD: u32 = 512;
+
 /// Fetch the nonce of the given `account` from the chain state.
 ///
 /// Note: Should only be used for tests.
@@ -193,6 +197,7 @@ pub fn new_partial(
 
 	let (grandpa_block_import, grandpa_link) = grandpa::block_import(
 		client.clone(),
+		GRANDPA_JUSTIFICATION_PERIOD,
 		&(client.clone() as Arc<_>),
 		select_chain.clone(),
 		telemetry.as_ref().map(|x| x.handle()),
@@ -524,7 +529,7 @@ pub fn new_full_base(
 	let grandpa_config = grandpa::Config {
 		// FIXME #1578 make this available through chainspec
 		gossip_duration: std::time::Duration::from_millis(333),
-		justification_period: 512,
+		justification_generation_period: GRANDPA_JUSTIFICATION_PERIOD,
 		name: Some(name),
 		observer_enabled: false,
 		keystore,