Add generic reverse proxy worker so that module-driven reverse proxying works....

Add generic reverse proxy worker so that module-driven reverse proxying works. It is enabled by default, but IMO it should be configurable like any other worker so we can limit the number of outgoing connections.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@109316 13f79535-47bb-0310-9956-ffa450edef68