Modify mod_file_cache to save pre-formatted strings for

content-length and last-modified headers for performance.

Submitted by:  Mike Abbot <mja@trudge.engr.sgi.com>
Reviewed by: Bill Stoddard


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88292 13f79535-47bb-0310-9956-ffa450edef68