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
Do not enqueue on cdb queue if not initialized
(cherry picked from commit 8fc10679c4a793518165e3f32f288ced29824fdb)
Remove assert when cdbq enqueue fails
(cherry picked from commit 9541631a89c5447e3b80a9e049898a97e0d5ac55)
Merge pull request #497 from Juniper/meghb_2488

Fix for bug 2488, 2667, 2672, 2675