Avoid using large pass-by-value struct arguments in pgbench.

Avoid using large pass-by-value struct arguments in pgbench.

In the wake of commit 4a39f87ac, which noticeably increased the
size of struct StatsData and thereby ParsedScript, Coverity started
to complain that ParsedScript was unreasonably large to be passing
by value.  The two places that do this are only used during setup,
so they're not really dragging down benchmark measurements --- but
gratuitous inefficiency is not a good look in a benchmarking program.
Convert to use pointers instead.