mod_cache: Add a discrete commit_entity() provider function within the

mod_cache provider interface which is called to indicate to the
provider that caching is complete, giving the provider the opportunity
to commit temporary files permanently to the cache in an atomic
fashion. Move all "rename" functionality of temporary files to permanent
files within mod_disk_cache from ad hoc locations in the code to the
commit_entity() function. Instead of reusing the same variables for
temporary file handling in mod_disk_cache, introduce separate discrete
structures for each of the three cache file types, the headers file,
vary file and data file, so that the atomic rename of all three file
types within commit_entity() becomes possible. Replace the inconsistent
use of error cleanups with a formal set of pool cleanups attached to
a subpool, which is destroyed on error.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@997545 13f79535-47bb-0310-9956-ffa450edef68