postgres_fdw: Push down partition-wise aggregation.

Since commit 7012b132d0, postgres_fdw
has been able to push down the toplevel aggregation operation to the
remote server.  Commit e2f1eb0ee3 made
it possible to break down the toplevel aggregation into one
aggregate per partition.  This commit lets postgres_fdw push down
aggregation in that case just as it does at the top level.

In order to make this work, this commit adds an additional argument
to the GetForeignUpperPaths FDW API.  A matching argument is added
to the signature for create_upper_paths_hook.  Third-party code using
either of these will need to be updated.

Also adjust create_foreignscan_plan() so that it picks up the correct
set of relids in this case.

Jeevan Chalke, reviewed by Ashutosh Bapat and by me and with some
adjustments by me.  The larger patch series of which this patch is a
part was also reviewed and tested by Antonin Houska, Rajkumar
Raghuwanshi, David Rowley, Dilip Kumar, Konstantin Knizhnik, Pascal
Legrand, and Rafia Sabih.

Discussion: http://postgr.es/m/CAM2+6=V64_xhstVHie0Rz=KPEQnLJMZt_e314P0jaT_oJ9MR8A@mail.gmail.com
Discussion: http://postgr.es/m/CAM2+6=XPWujjmj5zUaBTGDoB38CemwcPmjkRy0qOcsQj_V+2sQ@mail.gmail.com

doc/src/sgml/fdwhandler.sgml

@@ -359,7 +359,8 @@ void
 GetForeignUpperPaths(PlannerInfo *root,
                      UpperRelationKind stage,
                      RelOptInfo *input_rel,
-                     RelOptInfo *output_rel);
+                     RelOptInfo *output_rel,
+                     void *extra);
 </programlisting>
      Create possible access paths for <firstterm>upper relation</firstterm> processing,
      which is the planner's term for all post-scan/join query processing, such
@@ -379,7 +380,11 @@ GetForeignUpperPaths(PlannerInfo *root,
      currently being considered.  <literal>output_rel</literal> is the upper relation
      that should receive paths representing computation of this step,
      and <literal>input_rel</literal> is the relation representing the input to this
-     step.  (Note that <structname>ForeignPath</structname> paths added
+     step.  The <literal>extra</literal> parameter provides additional details,
+     currently, it is set only for <literal>UPPERREL_PARTIAL_GROUP_AGG</>
+     or <literal>UPPERREL_GROUP_AGG</literal>, in which case it points to a
+     <literal>GroupPathExtraData</> structure.
+     (Note that <structname>ForeignPath</structname> paths added
      to <literal>output_rel</literal> would typically not have any direct dependency
      on paths of the <literal>input_rel</literal>, since their processing is expected
      to be done externally.  However, examining paths previously generated for