Add force recovery for balancer.

In case all balancer members were in error state 503
was returned until the recovery timeout expired.
The patch forces recovery in case all balancer members
are in error state regardless of recovery timeout
directive.
This fixes the time gap when 503 was returned and
backend was already up and running.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@451572 13f79535-47bb-0310-9956-ffa450edef68