Fix up parallel-safety marking for appendrels.

The previous coding assumed that the value derived by
set_rel_consider_parallel() for an appendrel parent would be accurate for
all the appendrel's children; but this is not so, for example because one
child might scan a temp table.  Instead, apply set_rel_consider_parallel()
to each child rel as well as the parent, and then take the AND of the
results as controlling parallel safety for the appendrel as a whole.

(We might someday be able to deal more intelligently than this with cases
in which some of the childrels are parallel-safe and others not, but that's
for later.)

Robert Haas and Tom Lane