Add canceling waiting composes
    
    Extend the DELETE method handler to allow users to cancel their own
    waiting composes. Admins can cancel anything. Deleting data is still
    limited to admins only.
    
    It is possible that while the revocation is happening a worker will take
    the compose task, in which case it will continue and finish. It's thus
    possible for compose to go wait -> failed -> done.
    
    Cancelling an older compose without stored task ID will only mark it as
    failed with no other change.
    
    JIRA: COMPOSE-4168