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/service/src/task_manager.rs

@@ -15,10 +15,10 @@
 
 use std::{panic, pin::Pin, result::Result, sync::Arc};
 use exit_future::Signal;
-use log::{debug};
+use log::debug;
 use futures::{
 	Future, FutureExt,
-	future::{select, Either},
+	future::{select, Either, BoxFuture},
 	compat::*,
 	task::{Spawn, FutureObj, SpawnError},
 };
@@ -62,7 +62,12 @@ impl SpawnTaskHandle {
 	}
 
 	/// Helper function that implements the spawning logic. See `spawn` and `spawn_blocking`.
-	fn spawn_inner(&self, name: &'static str, task: impl Future<Output = ()> + Send + 'static, task_type: TaskType) {
+	fn spawn_inner(
+		&self,
+		name: &'static str,
+		task: impl Future<Output = ()> + Send + 'static,
+		task_type: TaskType,
+	) {
 		let on_exit = self.on_exit.clone();
 		let metrics = self.metrics.clone();
 
@@ -119,6 +124,12 @@ impl Spawn for SpawnTaskHandle {
 	}
 }
 
+impl sp_core::traits::SpawnBlocking for SpawnTaskHandle {
+	fn spawn_blocking(&self, name: &'static str, future: BoxFuture<'static, ()>) {
+		self.spawn_blocking(name, future);
+	}
+}
+
 impl sc_client_api::CloneableSpawn for SpawnTaskHandle {
 	fn clone(&self) -> Box<dyn CloneableSpawn> {
 		Box::new(Clone::clone(self))