Add destroy to a task's allowed_operations

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Move task_status_is_completed to TaskHelper

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>

Conflicts:
	ocaml/idl/ocaml_backend/taskHelper.ml
	ocaml/idl/ocaml_backend/taskHelper.mli
Don't destroy tasks that aren't complete

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Merge pull request #1811 from jonludlam/CA-130821-creedence

CA-130821 - better handling of Task.destroy API