delinvoice was orginally documented to only allow deletion of unpaid
invoices, but there might be reasons to delete paid ones or unexpired ones.
But we have to avoid the race where someone pays as it's deleted: the
easiest way is to have the caller tell us the status, and fail if
it's wrong.
Fixes: #477
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
