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/telemetry/src/lib.rs

@@ -27,7 +27,7 @@ extern crate slog_async;
 extern crate slog_json;
 #[macro_use]
 extern crate log;
-#[macro_use(o, kv)]
+#[macro_use(o)]
 extern crate slog;
 extern crate slog_scope;