Move spawning tasks from thread pools to Service's TaskManager for block importing (#5647)

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

substrate/client/service/src/config.rs

@@ -38,7 +38,7 @@ pub struct Configuration {
 	/// Node role.
 	pub role: Role,
 	/// How to spawn background tasks. Mandatory, otherwise creating a `Service` will error.
-	pub task_executor: Arc<dyn Fn(Pin<Box<dyn Future<Output = ()> + Send>>) + Send + Sync>,
+	pub task_executor: Arc<dyn Fn(Pin<Box<dyn Future<Output = ()> + Send>>, TaskType) + Send + Sync>,
 	/// Extrinsic pool configuration.
 	pub transaction_pool: TransactionPoolOptions,
 	/// Network configuration.
@@ -102,6 +102,15 @@ pub struct Configuration {
 	pub announce_block: bool,
 }
 
+/// Type for tasks spawned by the executor.
+#[derive(PartialEq)]
+pub enum TaskType {
+	/// Regular non-blocking futures. Polling the task is expected to be a lightweight operation.
+	Async,
+	/// The task might perform a lot of expensive CPU operations and/or call `thread::sleep`.
+	Blocking,
+}
+
 /// Configuration of the client keystore.
 #[derive(Clone)]
 pub enum KeystoreConfig {