Test calls to CDB_Invalidate_Varnish

Also drops the DELETE trigger on CDB_TableMetadata as the call to
CDB_Invalidate_Varnish would then get an OID, which is pretty useless.
Closes #8

scripts-available/CDB_TableMetadata.sql

@@ -94,7 +94,9 @@ $$
 LANGUAGE plpgsql VOLATILE SECURITY DEFINER;
 
 DROP TRIGGER IF EXISTS table_modified ON CDB_TableMetadata;
-CREATE TRIGGER table_modified AFTER INSERT OR UPDATE OR DELETE
+-- NOTE: on DELETE we would be unable to convert the table
+--       oid (regclass) to its name
+CREATE TRIGGER table_modified AFTER INSERT OR UPDATE
 ON CDB_TableMetadata FOR EACH ROW EXECUTE PROCEDURE
  _CDB_TableMetadata_Updated();