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/cli/src/lib.rs

@@ -50,7 +50,6 @@ extern crate clap;
 extern crate error_chain;
 #[macro_use]
 extern crate log;
-#[macro_use]
 extern crate structopt;
 
 mod params;