Starting and aborting transactions in security definer procedures
doesn't work.  StartTransaction() insists that the security context
stack is empty, so this would currently cause a crash, and
AbortTransaction() resets it.  This could be made to work by
reorganizing the code, but right now we just prohibit it.
Reported-by: amul sul <sulamul@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b96Gupt_LFL7uNyy3c50-wbhA68NUjiK5%3DrF6_w%3Dpq_T%3DQ%40mail.gmail.com