Rewrite sync and network layers to use futures (#2683)

* Remove background thread from network

* Primarily use tokio-timer instead of tokio

* Minor style fix

* Reintroduce tokio as an optional dependency

* Renames

* Line width

* Ctxt -> Context

* Merge NetworkWorker and NetworkService

substrate/core/service/src/lib.rs

@@ -203,7 +203,13 @@ impl<Components: components::Components> Service<Components> {
 		};
 
 		let has_bootnodes = !network_params.network_config.boot_nodes.is_empty();
-		let network = network::Service::new(network_params)?;
+		let network_mut = network::NetworkWorker::new(network_params)?;
+		let network = network_mut.service().clone();
+
+		task_executor.spawn(network_mut
+			.map_err(|_| ())
+			.select(exit.clone())
+			.then(|_| Ok(())));
 
 		let inherents_pool = Arc::new(InherentsPool::default());
 		let offchain_workers =  if config.offchain_worker {