The root cause for vizd crash in http://10.84.5.133/bugs/show_bug.cgi?id=2811 is CdbIf::CleanupTask was enqueued twice with same task pointer due to concurrent threads executing DB_Uninit

To detect this problem state sooner, an assert if the Task pointer is enqueued twice.
Merge pull request #605 from Juniper/double_enqueue

Assert on duplicate enqueue of same task pointer