CBQE-0: fix task manager cancel shotdown - _getitem_ issue

Change-Id: Iafe176d0449cff36a26525012eefc31ca79201d0
Reviewed-on: http://review.couchbase.org/35738
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>