mod_proxy_http: Add detach_backend hook.

The immediate use is for a SSL-related module which works
on the backend proxy connection to be able to "leak" information
into the client r for logging.

This could be useful with other proxy scheme handlers.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1586719 13f79535-47bb-0310-9956-ffa450edef68