Expose that BasicQueue expects blocking spawn (#5860)

* Expose that `BasicQueue` expects blocking spawn

Up to now `BasicQueue` expected a closure that to spawn a `Future`.
This was expected to be a closure that spawns a blocking future.
However, this wasn't documented anywhere. This pr introduces a new trait
`SpawnBlocking` that exposes this requirement to the outside.

* Feedback

substrate/client/network/src/service/tests.rs

@@ -80,15 +80,12 @@ fn build_test_full_node(config: config::NetworkConfiguration)
 		}
 	}
 
-	let threads_pool = futures::executor::ThreadPool::new().unwrap();
-	let spawner = |future| threads_pool.spawn_ok(future);
-
 	let import_queue = Box::new(sp_consensus::import_queue::BasicQueue::new(
 		PassThroughVerifier(false),
 		Box::new(client.clone()),
 		None,
 		None,
-		spawner,
+		&sp_core::testing::SpawnBlockingExecutor::new(),
 	));
 
 	let worker = NetworkWorker::new(config::Params {