Re: Allowing parallel-safe initplans

I tried both of those and concluded they'd be too messy for a patch
that we might find ourselves having to back-patch.  So 0001 attached
fixes it by teaching SS_finalize_plan to treat optimized MIN()/MAX()
aggregates as if they were already Params.  It's slightly annoying
to have knowledge of that optimization metastasizing into another
place, but the alternatives are even less palatable.

I'm still resistant to the idea of kluging EXPLAIN to the extent
of hiding the EXPLAIN output changes.  It wouldn't be that hard
to do really, but I worry that such a kluge might hide real problems
in future.  So what I did in 0002 was to allow initPlans for an
injected Gather only if debug_parallel_query = on, so that there
will be a place for EXPLAIN to show them.  Other than the changes
in that area, 0002 is the same as the previous patch.