GRANDPA: announce blocks we vote on to peers (#1593)

* announce blocks we vote on to peers (missing network impl)

* Implemented 'announce'

* Fixed test

* improve announce docs and add logging

* Track recently announced blocks

substrate/core/network/src/service.rs

@@ -200,6 +200,14 @@ impl<B: BlockT + 'static, S: NetworkSpecialization<B>, H: ExHashT> Service<B, S,
 		self.handler.propagate_extrinsics(&mut NetSyncIo::new(&self.network, self.protocol_id));
 	}
 
+	/// Make sure an important block is propagated to peers.
+	///
+	/// In chain-based consensus, we often need to make sure non-best forks are
+	/// at least temporarily synced.
+	pub fn announce_block(&self, hash: B::Hash) {
+		self.handler.announce_block(&mut NetSyncIo::new(&self.network, self.protocol_id), hash);
+	}
+
 	/// Send a consensus message through the gossip
 	pub fn gossip_consensus_message(&self, topic: B::Hash, message: Vec<u8>, broadcast: bool) {
 		self.handler.gossip_consensus_message(