Merge r417238, r437773, r446929, r453630 from trunk:

Merge r417238, r437773, r446929, r453630 from trunk:

* Add the following environment variables to expose the information
* about
  the route, the sticky session and the worker used during a request to
  other modules:

  BALANCER_SESSION_STICKY
  BALANCER_SESSION_ROUTE
  BALANCER_NAME
  BALANCER_WORKER_NAME
  BALANCER_WORKER_ROUTE

PR: 39806
Submitted by: Brian <brectanu gmail.com>
Reviewed by: rpluem

fix validation error

* Set the new environment variable BALANCER_ROUTE_CHANGED if a worker with a
  route different from the one supplied by the client had been chosen or if
  the client supplied no routing information for a balancer with sticky
  sessions.

* mod_proxy_balancer: Document BALANCER_ROUTE_CHANGED environment variable.

Submitted by: Brian Rectanus <brectanu gmail.com>
Reviewed by: rpluem

Reviewed by: jim

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@472114 13f79535-47bb-0310-9956-ffa450edef68