Push scan/join target list beneath Gather when possible.

Push scan/join target list beneath Gather when possible.

This means that, for example, "SELECT expensive_func(a) FROM bigtab
WHERE something" can compute expensive_func(a) in the workers rather
than the leader if it happens to be parallel-safe, which figures to be
a big win in some practical cases.

Currently, we can only do this if the entire target list is
parallel-safe.  If we worked harder, we might be able to evaluate
parallel-safe targets in the worker and any parallel-restricted
targets in the leader, but that would be more complicated, and there
aren't that many parallel-restricted functions that people are likely
to use in queries anyway.  I think.  So just do the simple thing for
the moment.

Robert Haas, Amit Kapila, and Tom Lane