Identity columns

This is the SQL standard-conforming variant of PostgreSQL's serial
columns.  It fixes a few usability issues that serial columns have:

- CREATE TABLE / LIKE copies default but refers to same sequence
- cannot add/drop serialness with ALTER TABLE
- dropping default does not drop sequence
- need to grant separate privileges to sequence
- other slight weirdnesses because serial is some kind of special macro

Reviewed-by: Vitaly Burovoy <vitaly.burovoy@gmail.com>

src/backend/nodes/readfuncs.c

@@ -247,6 +247,7 @@ _readQuery(void)
 	READ_NODE_FIELD(rtable);
 	READ_NODE_FIELD(jointree);
 	READ_NODE_FIELD(targetList);
+	READ_ENUM_FIELD(override, OverridingKind);
 	READ_NODE_FIELD(onConflict);
 	READ_NODE_FIELD(returningList);
 	READ_NODE_FIELD(groupClause);