Switch to the new 32-bit APR atomic API for better portability

(the old code assumed that apr_atomic_t and apr_uint32_t were interchangeable).
Also, add more detailed comments on how one of the synchronization
functions works.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101340 13f79535-47bb-0310-9956-ffa450edef68