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]

rpc/src/v1/tests/helpers/sync_provider.rs

@@ -51,6 +51,7 @@ impl TestSyncProvider {
 				mem_used: 0,
 				num_snapshot_chunks: 0,
 				snapshot_chunks_done: 0,
+				last_imported_old_block_number: None,
 			}),
 		}
 	}