core: Remove apr_brigade_flatten(), buffering and duplicated code

from the HTTP_IN filter, parse chunks in a single pass with zero copy.
Reduce memory usage by 48 bytes per request.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1484852 13f79535-47bb-0310-9956-ffa450edef68