Experimental cache based on Graham Leggett's layered cache design. mod_cache

Experimental cache based on Graham Leggett's layered cache design.  mod_cache
implements a quick handler, and three filters. The filters are
CACHE_IN for loading the cache, CACHE_OUT for serving content out of the cache
and CACHE_CONDITIONAL, which handles stale entries in the cache.

mod_cache implements code that makes RFC compliant caching decisions. It
interfaces with the actual storage mechanism via calls to functions defined in
cache_storage.c.  This commit includes a simple in memory (malloc'ed memory)
cache implementation that demonstrates autoloading and serving files
keyed on URL.

This is not even close to production ready. You have been warned :-)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90549 13f79535-47bb-0310-9956-ffa450edef68