Sync: Propagate block announcement data (#7903)

* Sync: Propagate block announcement data

This pr adds a feature to the sync protocol to propagate the data that
we received alongside a block announcement. This is done by adding a
cache that caches the last X block announcement data where X is set to
the number of `in_peers` (giving every peer the chance to send us a
different block). This will be required by parachains to ensure that
even peers who are not connected to a collator receive the data
alongside the block announcement to properly validate it and request the
block.

* Review comment

* Bring back the code and add new variant to ensure we don't insert block
announce data when something wasn't checked

* Also use out_peers

substrate/client/network/src/service.rs

@@ -858,7 +858,7 @@ impl<B: BlockT + 'static, H: ExHashT> NetworkService<B, H> {
 	///
 	/// In chain-based consensus, we often need to make sure non-best forks are
 	/// at least temporarily synced. This function forces such an announcement.
-	pub fn announce_block(&self, hash: B::Hash, data: Vec<u8>) {
+	pub fn announce_block(&self, hash: B::Hash, data: Option<Vec<u8>>) {
 		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::AnnounceBlock(hash, data));
 	}
 
@@ -1236,7 +1236,7 @@ enum ServiceToWorkerMsg<B: BlockT, H: ExHashT> {
 	PropagateTransaction(H),
 	PropagateTransactions,
 	RequestJustification(B::Hash, NumberFor<B>),
-	AnnounceBlock(B::Hash, Vec<u8>),
+	AnnounceBlock(B::Hash, Option<Vec<u8>>),
 	GetValue(record::Key),
 	PutValue(record::Key, Vec<u8>),
 	AddKnownAddress(PeerId, Multiaddr),