Rewrite the BasiQueue using channels (#1327)

* use channels to implement basic import queue

* async justification import

* better conditional for is_done in tests

* reword the test for presence of link

* fix conditional

* trace instead of panic when no link present

* reword expectations when sending to importers

* fix

* debug justification import error

* update expectations

* use NumberFor

* nits

* add general description

* move error handling into closure

substrate/core/service/src/lib.rs

@@ -123,7 +123,7 @@ impl<Components: components::Components> Service<Components> {
 		};
 
 		let (client, on_demand) = Components::build_client(&config, executor)?;
-		let import_queue = Arc::new(Components::build_import_queue(&mut config, client.clone())?);
+		let import_queue = Box::new(Components::build_import_queue(&mut config, client.clone())?);
 		let best_header = client.best_block_header()?;
 
 		let version = config.full_version();