Improve efficiency of http response writer

Writing http results currently involves first writing into a request buffer and then copying this buffer into the http ResponseWriter buffer.
While the request buffer is pooled, in principle it could grow up to a KeepAlive server parameter size (usually 32K), but it starts at 512 bytes.
This means that we have still a fair amount of growth and reallocation on one buffer, a copy, and no control on the other buffer.
Ideally we should only have one buffer sized to KeepAlive, no copying, no growth, and flushing as required.