Instead of performing a proper join on tabname, CDB_TableMetadata_Text joins
cdb_tablemetadata against pg_catalog.pg_class (i.e. All postgres tables,
views, indices, etc.) and gives a record for every possible tabname and
updated_at combination. This results in the latest updated timestamp being
chosen for any table in CDB_Get_Foreign_Updated_At, which leads to unnecessary
and incorrect cache invalidation.
NOTE: also makes CDB_TableMetadata triggers still functional
when user quota and varnish invalidation functions are missing
from the database (currently installed by cartodb ruby)