mod_proxy: Make ap_proxy_retry_worker() into an optional function. Allows

mod_lbmethod_bybusyness, mod_lbmethod_byrequests, mod_lbmethod_bytraffic
and mod_lbmethod_heartbeat to be loaded without mod_proxy yet being present,
which happens when modules are loaded in alphabetical order.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1209754 13f79535-47bb-0310-9956-ffa450edef68