mod_disk_cache: Change on-disk header file format to support the

link of the device/inode of the data file to the matching header
file, and to support the option of not writing a data file when
the data file is empty. Refactor the mod_disk_cache code so that
headers are written as late as possible (on commit), allowing the
device and inode of the body to be written to the header. At this
point, writes to the cache are now atomic, without locks.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1002643 13f79535-47bb-0310-9956-ffa450edef68