Generate a separate upper relation for each stage of setop planning.

Commit 3fc6e2d7 made setop planning
stages return paths rather than plans, but all such paths were loosely
associated with a single RelOptInfo, and only the final path was added
to the RelOptInfo.  Even at the time, it was foreseen that this should
be changed, because there is otherwise no good way for a single stage
of setop planning to return multiple paths.  With this patch, each
stage of set operation planning now creates a separate RelOptInfo;
these are distinguished by using appropriate relid sets.  Note that
this patch does nothing whatsoever about actually returning multiple
paths for the same set operation; it just makes it possible for a
future patch to do so.

Along the way, adjust things so that create_upper_paths_hook is called
for each of these new RelOptInfos rather than just once, since that
might be useful to extensions using that hook.  It might be a good
to provide an FDW API here as well, but I didn't try to do that for
now.

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