jqueue: Move CancelJob logic to separate function

Moving the internals of this function will allow it to be used from
unittests in the future. Splitting this into a pure, side-effect free
function and an impure one makes the pure function easily testable.

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