add a optional function ap_cache_generate_key

which allows a 3rd party module to generate the key name based
on the request_rec.

the idea here is for it to also be able to mess with expiry times
and cachability

add another option.. CacheIgnoreCacheControl. this ignores a 'incoming request's
attempts to get a fresh copy. Mainly I see this as being usefull in r-proxy's


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93242 13f79535-47bb-0310-9956-ffa450edef68