LP#1270289 Support canceling already-canceled lineitems

LP#1270289 Support canceling already-canceled lineitems

If a lineitem is canceled and the cancel_reason has keep_debits == true
(i.e. the lineitem is "delayed"), allow the lineitem to be canceled
again, up until the point the lineitem cancel_reason has keep_debits ==
false, at which point, no more cancelations are allowed.

Similarly, when a copy is canceled with a keep_debits==true reason,
allow the copy to be canceled when its parent lineitem is canceled,
following the rules above.

Signed-off-by: Bill Erickson <berick@esilibrary.com>