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/src/standard.rs

@@ -46,4 +46,4 @@ pub use rustc_serialize::hex::{FromHex, FromHexError};
 pub use heapsize::HeapSizeOf;
 pub use itertools::Itertools;
 
-pub use parking_lot::{Condvar, Mutex, MutexGuard, RwLock, RwLockReadGuard, RwLockWriteGuard};
+pub use parking_lot::{Condvar, Mutex, MutexGuard, RwLock, RwLockReadGuard, RwLockWriteGuard};