[MINOR] stats: use a dedicated state to output static data

[MINOR] stats: use a dedicated state to output static data

It is a bit expensive and complex to use to call buffer_feed()
directly from the request parser, and there are risks that some
output messages are lost in case of buffer full. Since most of
these messages are static, let's have a state dedicated to print
these messages and store them in a specific area shared with the
stats in the session. This both reduces code size and risks of
losing output data.