Use executor instead of thread

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

@@ -10,6 +10,8 @@ import com.sun.mail.imap.IMAPStore;
 
 import java.util.Date;
 import java.util.Properties;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 
 import javax.mail.Folder;
 import javax.mail.Message;
@@ -33,6 +35,7 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
     private String search;
     private Handler mainHandler;
     private IBoundaryCallbackMessages intf;
+    private ExecutorService executor = Executors.newSingleThreadExecutor();
 
     private boolean enabled = false;
     private IMAPStore istore = null;
@@ -91,10 +94,9 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
     }
 
     void load(final long before) {
-        new Thread(new Runnable() {
+        executor.submit(new Runnable() {
             @Override
             public void run() {
-                synchronized (context.getApplicationContext()) {
                 try {
                     mainHandler.post(new Runnable() {
                         @Override
@@ -166,7 +168,6 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
                     });
                 }
             }
-            }
-        }).start();
+        });
     }
 }