Add a relkind field to RangeTblEntry to avoid some syscache lookups.

The recent additions for FDW support required checking foreign-table-ness
in several places in the parse/plan chain.  While it's not clear whether
that would really result in a noticeable slowdown, it seems best to avoid
any performance risk by keeping a copy of the relation's relkind in
RangeTblEntry.  That might have some other uses later, anyway.
Per discussion.

src/backend/nodes/outfuncs.c

@@ -2275,8 +2275,8 @@ _outRangeTblEntry(StringInfo str, RangeTblEntry *node)
 	switch (node->rtekind)
 	{
 		case RTE_RELATION:
-		case RTE_SPECIAL:
 			WRITE_OID_FIELD(relid);
+			WRITE_CHAR_FIELD(relkind);
 			break;
 		case RTE_SUBQUERY:
 			WRITE_NODE_FIELD(subquery);