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/equalfuncs.c

@@ -2286,6 +2286,7 @@ _equalRangeTblEntry(RangeTblEntry *a, RangeTblEntry *b)
 {
 	COMPARE_SCALAR_FIELD(rtekind);
 	COMPARE_SCALAR_FIELD(relid);
+	COMPARE_SCALAR_FIELD(relkind);
 	COMPARE_NODE_FIELD(subquery);
 	COMPARE_SCALAR_FIELD(jointype);
 	COMPARE_NODE_FIELD(joinaliasvars);