postgres_fdw: Perform the (FINAL, NULL) upperrel operations remotely.

The upper-planner pathification allows FDWs to arrange to push down
different types of upper-stage operations to the remote side.  This
commit teaches postgres_fdw to do it for the (FINAL, NULL) upperrel,
which is responsible for doing LockRows, LIMIT, and/or ModifyTable.
This provides the ability for postgres_fdw to handle SELECT commands
so that it 1) skips the LockRows step (if any) (note that this is
safe since it performs early locking) and 2) pushes down the LIMIT
and/or OFFSET restrictions (if any) to the remote side.  This doesn't
handle the INSERT/UPDATE/DELETE cases.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska and Jeff Janes
Discussion: https://postgr.es/m/87pnz1aby9.fsf@news-spur.riddles.org.uk

src/include/nodes/pathnodes.h

@@ -2439,6 +2439,24 @@ typedef struct
 	PartitionwiseAggregateType patype;
 } GroupPathExtraData;
 
+/*
+ * Struct for extra information passed to subroutines of grouping_planner
+ *
+ * limit_needed is true if we actually need a Limit plan node
+ * limit_tuples is an estimated bound on the number of output tuples,
+ *		or -1 if no LIMIT or couldn't estimate
+ * count_est and offset_est are the estimated values of the LIMIT and OFFSET
+ * 		expressions computed by preprocess_limit() (see comments for
+ * 		preprocess_limit() for more information).
+ */
+typedef struct
+{
+	bool		limit_needed;
+	double		limit_tuples;
+	int64		count_est;
+	int64		offset_est;
+} FinalPathExtraData;
+
 /*
  * For speed reasons, cost estimation for join paths is performed in two
  * phases: the first phase tries to quickly derive a lower bound for the