PDO: Store/pass query_string as zend_string

Rather than storing char* + size_t, use a zend_string*. Also
avoid various copies of the query string.

ext/pdo_pgsql/php_pdo_pgsql_int.h

@@ -60,7 +60,7 @@ typedef struct {
 	pdo_pgsql_column        *cols;
 	char *cursor_name;
 	char *stmt_name;
-	char *query;
+	zend_string *query;
 	char **param_values;
 	int *param_lengths;
 	int *param_formats;