Make proxy_conn always accesible for preforked MPM and

APR_HAS_THREADS enabled.

Add init_conn_worker for creating apr_reslist that maintains connection
pool. For prefork mpm's use the single connection container.

Submitted by: mturk


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@104585 13f79535-47bb-0310-9956-ffa450edef68