Fixed mod_mem_cache so that it doesn't misfile entries in the priority

Fixed mod_mem_cache so that it doesn't misfile entries in the priority
queue.  Previously, since the ->priority value was positive, certain
parts of mod_mem_cache would cause the priority queue to do the wrong thing
when changing the priority of an object in the cache.  The end result
would be that the objects would not be dequeued in the right order.

Submitted by:  Jean-Jacques Clar, Cliff Woolley
Reviewed by:   Paul J. Reder

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101958 13f79535-47bb-0310-9956-ffa450edef68