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/verification/verification.rs

@@ -296,7 +296,7 @@ mod tests {
 			self.blocks.contains_key(hash)
 		}
 
-		fn first_block(&self) -> H256 {
+		fn first_block(&self) -> Option<H256> {
 			unimplemented!()
 		}
 
@@ -313,6 +313,10 @@ mod tests {
 			self.block(hash).map(|b| BlockChain::block_to_body(&b))
 		}
 
+		fn best_ancient_block(&self) -> Option<H256> {
+			None
+		}
+
 		/// Get the familial details concerning a block.
 		fn block_details(&self, hash: &H256) -> Option<BlockDetails> {
 			self.blocks.get(hash).map(|bytes| {