Allocate request object from its own pool.

Previously, it was allocated from a connection pool and
was selectively freed for an idle keepalive connection.

The goal is to put coupled things in one chunk of memory,
and to simplify handling of request objects.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5097 73f98a42-aea0-e011-b76d-00259023448c