mod_proxy_balancer: Restore balancing after a failed worker has
recovered when using lbmethod_bybusyness.

PR: 48735

Markus Stoll and Adam C both submitted patches against 2.2.x
to bug 48735.  Compared with those two, this solution

1. resets the busy field in the error-ed worker at the end of
   the request instead of at recovery time
2. leaves the lbstatus field alone
3. covers all possible scenarios where the busy field in the
   error-ed worker needs to be adjusted, since a cleanup to
   perform the decrement is registered at the point of the
   increment

Submitted by: trawick
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1366691 13f79535-47bb-0310-9956-ffa450edef68