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]

util/network/src/session.rs

@@ -30,7 +30,7 @@ use node_table::NodeId;
 use stats::NetworkStats;
 use time;
 
-const PING_TIMEOUT_SEC: u64 = 30;
+const PING_TIMEOUT_SEC: u64 = 65;
 const PING_INTERVAL_SEC: u64 = 30;
 
 /// Peer session over encrypted connection.