Replace AsyncExecutor with simple closure

This makes everything much simpler. The task type is now defined
directly on `Plugin`.

plugins/crossover/src/lib.rs

@@ -183,8 +183,7 @@ impl Plugin for Crossover {
         aux_outputs: Some(&["Band 1", "Band 2", "Band 3", "Band 4", "Band 5"]),
     };
 
-    type AsyncExecutor = ();
-    fn async_executor(&self) -> Self::AsyncExecutor {}
+    type BackgroundTask = ();
 
     fn params(&self) -> Arc<dyn Params> {
         self.params.clone()