mod_disk_cache: Implement binary format for on-disk header files.

Justin greatly extended Brian's patch and redid the rest of the storage format.

Submitted by:	Brian Akins <bakins web.turner.com>
Reviewed by:	Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@104511 13f79535-47bb-0310-9956-ffa450edef68