Enforce one-payment-per-xact-per-call

Enforce one-payment-per-xact-per-call

There is no legitimate reason for a transaction to receive more than
one payment per call to open-ils.circ open-ils.circ.money.payment,
but we have seen the staff client generate such a data structure. This
leads to seemingly duplicate payments, and is bad all around.  So,
we will enforce the restriction by taking only the first payment per
xact in the list of payments.

Signed-off-by: Mike Rylander <mrylander@gmail.com>