Introduce the notion of a multi part cache object. Part of the cache_object

Introduce the notion of a multi part cache object. Part of the cache_object
is common across any cache implementation, the other part is private to
the particular implementation (eg, mem_cache_object_t/mod_mem_cache).
Use a cache_handle_t allocated out of the request pool to hold references
to the callback functions and common cache object.

The cache_handle_t contains implementation specific callback functions and
a reference to a common cache_object_t.  The cache_object_t contains
a reference to an implementation specific cache object extension (mem_cache_object_t
for example).

All this simplifies managing the callback function pointers (don't want to
save them in each cache entry) and collections of cache_object_t keyed
to a single url.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90994 13f79535-47bb-0310-9956-ffa450edef68