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/commands/copy.c

@@ -763,6 +763,7 @@ DoCopy(const CopyStmt *stmt, const char *queryString)
 		rte = makeNode(RangeTblEntry);
 		rte->rtekind = RTE_RELATION;
 		rte->relid = RelationGetRelid(rel);
+		rte->relkind = rel->rd_rel->relkind;
 		rte->requiredPerms = required_access;
 
 		tupDesc = RelationGetDescr(rel);