Replace AsyncExecutor with simple closure

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

plugins/spectral_compressor/src/lib.rs

@@ -272,8 +272,7 @@ impl Plugin for SpectralCompressor {
 
     const SAMPLE_ACCURATE_AUTOMATION: bool = true;
 
-    type AsyncExecutor = ();
-    fn async_executor(&self) -> Self::AsyncExecutor {}
+    type BackgroundTask = ();
 
     fn params(&self) -> Arc<dyn Params> {
         self.params.clone()