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]

ethcore/src/snapshot/tests/blocks.rs

@@ -69,7 +69,7 @@ fn chunk_and_restore(amount: u64) {
 	// restore it.
 	let new_db = Arc::new(Database::open(&db_cfg, new_path.as_str()).unwrap());
 	let new_chain = BlockChain::new(Default::default(), &genesis, new_db.clone());
-	let mut rebuilder = BlockRebuilder::new(new_chain, amount).unwrap();
+	let mut rebuilder = BlockRebuilder::new(new_chain, new_db.clone(), amount).unwrap();
 	let reader = PackedReader::new(&snapshot_path).unwrap().unwrap();
 	let engine = ::engines::NullEngine::new(Default::default(), Default::default());
 	for chunk_hash in &reader.manifest().block_hashes {