Convert the jobqueue rpc to new style result

This patch converts the job queue rpc calls to the new style result.
It's done in a single patch as there are helper function (in both jqueue
and backend) that are used by multiple rpcs and need synchronized
change.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>