Consider Parallel Append of partial paths for UNION [ALL].

Consider Parallel Append of partial paths for UNION [ALL].

Without this patch, we can implement a UNION or UNION ALL as an
Append where Gather appears beneath one or more of the Append
branches, but this lets us put the Gather node on top, with
a partial path for each relation underneath.

There is considerably more work that could be done to improve
planning in this area, but that will probably need to wait
for a future release.

Patch by me, reviewed and tested by Ashutosh Bapat and Rajkumar
Raghuwanshi.

Discussion: http://postgr.es/m/CA+TgmoaLRAOqHmMZx=ESM3VDEPceg+-XXZsRXQ8GtFJO_zbMSw@mail.gmail.com