Upgrade lib/sql to latest cartodb-postgresql master

Changes affecting current cartodb:

 - Remove CDB_SearchPath.sql from the set of scripts loaded directly
   Closes #466
 - Do not call CDB_Invalidate_Varnish() if not owned by a superuser

Other changes:

 - Migrate user quota on cartodb extension creation from unpackaged
 - Complain if script is sourced in psql, rather than via CREATE EXTENSION
 - Refuse to create new extension if legacy code is present on database
 - Fix extension requires directive