Move pool maintainance to a background task. (#1236)

* Move pool maintainance to a background task.

* Remove commented code.

* Add issue number to TODOs

* Fix warnings and tests.

substrate/core/service/src/error.rs

@@ -24,6 +24,7 @@ error_chain! {
 	foreign_links {
 		Io(::std::io::Error) #[doc="IO error"];
 	}
+
 	links {
 		Client(client::error::Error, client::error::ErrorKind) #[doc="Client error"];
 		Network(network::error::Error, network::error::ErrorKind) #[doc="Network error"];