[perf] replace buffer in PartialResponseWriter implementation with a fast filter

The current implementation of PartialResponseWriter uses a StringWriter with an inner StringBuilder to store the data and then post-process it, escaping CDATA ending sections.

After checking the code I notice it is possible to replace the buffer with a Filter that escape such sections "on the fly". This will reduce the memory used to handle ajax requests and improve speed a little bit, because the jvm does not longer need to expand over and over such buffer to fit the response.