be consistant about how a couple of members of mem_cache_object_t are used

and defined to silence compile errors on a system where apr_atomic_t is
a structure.

"pos" is read & written with atomic operators, so make it an apr_atomic_t.
"priority" is almost always updated without using atomic operators, so I
assume it doesn't need to be atomic.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95597 13f79535-47bb-0310-9956-ffa450edef68