Use dedicated model executor

app/src/main/java/eu/faircode/email/ViewModelMessages.java

@@ -44,6 +44,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.concurrent.ExecutorService;
 
 public class ViewModelMessages extends ViewModel {
     private AdapterMessage.ViewType last = AdapterMessage.ViewType.UNIFIED;
@@ -67,6 +68,9 @@ public class ViewModelMessages extends ViewModel {
         }
     };
 
+    private static final ExecutorService executor =
+            Helper.getBackgroundExecutor(0, 0, 3, "model");
+
     private static final int LOCAL_PAGE_SIZE = 50;
     private static final int THREAD_PAGE_SIZE = 100;
     private static final int REMOTE_PAGE_SIZE = 10;
@@ -200,7 +204,7 @@ public class ViewModelMessages extends ViewModel {
                     break;
             }
 
-            builder.setFetchExecutor(db.getQueryExecutor());
+            builder.setFetchExecutor(executor);
 
             model = new Model(args, builder.build(), boundary);
             models.put(viewType, model);