Fix for bug 2488, 2667, 2672, 2675

1. Recreate db_connect_timer in generator every time the
   generator connects to vizd since the generator instance
   changes every time and we rely on the task instance to
   run the timer exclusive to the DB task.
2. Take cdbq_mutex when doing async column add to prevent
   against cdbq initialization happening concurrently.
3. When trying to run the cdb cleanup task, check if it is
   cancelled and if so just return
4.Do not enqueue on cdb queue if not initialized
5.Remove assert when cdbq enqueue fails