mod_proxy: add ap_proxy_define_match_worker() and use it for ProxyPassMatch

and ProxyMatch section to distinguish between normal workers and workers
with regex substitutions in the name. Implement handling of such workers
in ap_proxy_get_worker(). PR 43513


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1609680 13f79535-47bb-0310-9956-ffa450edef68