Workaround PostgreSQL bug possibly triggering infinite loops

Implements casting of existing system column values during
cartodbfication using ALTER rather than UPDATE.

It's faster for tables with many rows and works around a
longstanding PostgreSQL bug. See
http://www.postgresql.org/message-id/20140530143150.GA11051@localhost

CDB-3059

test/CDB_CartodbfyTableTest_expect

@@ -44,5 +44,8 @@ SELECT 1
 unsequenced cartodb_id cartodbfied fine
 1
 DROP TABLE
+CREATE TABLE
+null geom and timestamp values cartodbfied fine
+DROP TABLE
 DROP FUNCTION
 DROP FUNCTION