jqueue: Fix cancelling while in waitlock in queue

Since the recent change to leave jobs in the “waitlock” status (commit
5fd6b694), cancelling a job while it's back in the queue would break.
This patch handles these cases and adds a unittest.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>