Issue #2550693 by torotil, morbiD: Fix bogus entries in {paypal_payment_ipn}.

Also fixes:
- Issue #2555297 by morbiD: IPN's always have pid's of 0 in database

NOTE: This will delete all entries in {paypal_payment_ipn} with pid=0 (perhaps
all of them). If you still need those although they are not linked to
any particular payment you should make a backup of this table before
running the update.

{paypal_payment_ipn} is changed to store one transaction ID per payment
status item (instead of per payment). This allows multiple transaction IDs
to be stored for every payment.