Externalize the functions to manage the mpm's queue of completion_contexts.

Now a module can produce to the queue of contexts that the worker threads
consume from.

Insomnia tonight in Cary NC...


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92018 13f79535-47bb-0310-9956-ffa450edef68