Allow the core_output_filter to save data in the filter past the end of

Allow the core_output_filter to save data in the filter past the end of
the first request if this is a keepalive connection.  This allows us to
save the end of the first request until we have enough data from the
second request to make it worthwhile to send the responses.  In order
to do this, we need to allocate mmap's from the connection pool instead
of the request pool.  If we don't use the connection pool, then the mmap
is freed before the data is sent, because the mmap is freed at the end of
the first request.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86840 13f79535-47bb-0310-9956-ffa450edef68