Work in Progress.

Add Clustered proxying support to mod_serf, by using the heartbeats system.

No preconfiguration of cluster members is needed.

Just a config like this:
    SerfCluster sweet heartbeat file=/var/cache/apache/hb.dat
    SerfCluster sour heartbeat file=/var/cache/apache/cluster2.dat
    <Location "/">
      SerfPass cluster://sweet
    </Location>
    <Location "/different_cluster">
      SerfPass cluster://sour
    </Location>

The location of all possible destination servers is provided by a new 
providers interface, that includes configuration checking of the arguments to 
the SerfCluster command, solving one of the worst problems with the mod_proxy 
load balancer subsystem.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@759386 13f79535-47bb-0310-9956-ffa450edef68