Centralize hop-by-hop header handling in the disk caching code. Note that...

Centralize hop-by-hop header handling in the disk caching code. Note that 'out' now has actual out semantics; hence the change in headers_in.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@649176 13f79535-47bb-0310-9956-ffa450edef68