Revert private method for inheritance (#30)

Make shutdown logic accessible to classes that inherit this class.

src/main/java/com/salesforce/mirus/MirusSourceTask.java

@@ -121,7 +121,7 @@ public void stop() {
     }
   }
 
-  private void shutDownTask() {
+  protected void shutDownTask() {
     logger.debug("Task shutting down");
     consumer.close();
   }