Refactoring of ap_read_request() to store partial request state

in the request rec.  The point of this is to allow asynchronous
MPMs do do nonblocking reads of requests.  (Backported from the
async-read-dev branch)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@360461 13f79535-47bb-0310-9956-ffa450edef68