Merge r420936, r420954, r420986, r421283 from trunk:

Merge r420936, r420954, r420986, r421283 from trunk:

Restructure the find_route_worker() function. Basically,
it should take care of knowing about usable and unusable
workers. By centralizing this logic, it will make it
easier and more streamlined to add in the forthcoming
hot-standby status workers.

Clean up some proxy macros. Avoid the use of magic
numbers, and instead use pre-defined defines. Also,
ensure that usable workers have been initialized :)

Allocate a bit for hot standbys. Adjust so that
normal "usable" workers don't count these.

Add in hot-standby balancer member. If all other members
are disabled or not-usable, ONLY THEN will the hot
standby's be used.

Reviewed by: jim

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@438563 13f79535-47bb-0310-9956-ffa450edef68