Add two new modules to handle load balancing across multiple apache servers

Add two new modules  to handle load balancing across multiple apache servers
within the same datacenter.

mod_heartbeat generates multicast status messages with the current number of
clients connected, but the formated can easily be extended to include other
things.

mod_heartmonitor collects these messages into a static file, which then can be
used for other modules to make load balancing decisions on.

This module was originally written at Joost by Sander Striker, Justin
Erenkrantz, and myself.  We have been given permission by our employer to
contribute this module.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@721952 13f79535-47bb-0310-9956-ffa450edef68