Snapshot sync part 2 (#2098)

* Split block downloader into a module

* Snapshot sync progress

* Warp sync CLI option

* Increased snapshot chunk and ping timeouts

* Fixed an issue with delayed writes

* Updated bootnodes

* Don't run pending IO tasks on shutdown

* Optional first_block; removed insert_snapshot_block

* Fixing expect calls

* Fixed stalled sync

* style and docs

* Update block_sync.rs

[ci:skip]

sync/src/tests/helpers.rs

@@ -30,6 +30,7 @@ pub struct TestIo<'p> {
 	pub queue: &'p mut VecDeque<TestPacket>,
 	pub sender: Option<PeerId>,
 	pub to_disconnect: HashSet<PeerId>,
+	overlay: RwLock<HashMap<BlockNumber, Bytes>>,
 }
 
 impl<'p> TestIo<'p> {
@@ -40,6 +41,7 @@ impl<'p> TestIo<'p> {
 			queue: queue,
 			sender: sender,
 			to_disconnect: HashSet::new(),
+			overlay: RwLock::new(HashMap::new()),
 		}
 	}
 }
@@ -90,6 +92,10 @@ impl<'p> SyncIo for TestIo<'p> {
 	fn eth_protocol_version(&self, _peer: PeerId) -> u8 {
 		64
 	}
+
+	fn chain_overlay(&self) -> &RwLock<HashMap<BlockNumber, Bytes>> {
+		&self.overlay
+	}
 }
 
 pub struct TestPacket {
@@ -149,6 +155,7 @@ impl TestNet {
 			for client in 0..self.peers.len() {
 				if peer != client {
 					let mut p = self.peers.get_mut(peer).unwrap();
+					p.sync.write().restart(&mut TestIo::new(&mut p.chain, &p.snapshot_service, &mut p.queue, Some(client as PeerId)));
 					p.sync.write().on_peer_connected(&mut TestIo::new(&mut p.chain, &p.snapshot_service, &mut p.queue, Some(client as PeerId)), client as PeerId);
 				}
 			}