Response size clamp (#786)

*) Limit response http body or json response size to FN_MAX_RESPONSE_SIZE (default unlimited)
*) If limits are exceeded 502 is returned with 'body too large' in the error message

api/agent/func_logger.go

@@ -169,6 +169,7 @@ func (li *lineWriter) Close() error {
 }
 
 // io.Writer that allows limiting bytes written to w
+// TODO change to use clamp writer, this is dupe code
 type limitWriter struct {
 	n, max int
 	io.Writer