1. Generator database timer creation/deletion is done from
   generator connect session, disconnect session, database error
   handler. These can happen concurrently, synchronize by taking
   per generator mutex.
2. Generator db handler init, shutdown, and dequeue can happen
   concurrently. Enqueue a task to perform the init and ensure
   that these do not happen concurrently.
3. Cdb if now supports only_sync option that is used by
   query engine and when used does not create the cdbq.
QueryEngine uses only_sync cdbif
Merge pull request #570 from Juniper/meghb_2773

1. Generator database timer creation/deletion is done from