mod_proxy_balancer: Add hot spare member type and corresponding flag (R). Hot spare members are

used as drop-in replacements for unusable workers in the same load balancer set. This differs
from hot standbys which are only used when all workers in a set are unusable. PR 61140.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1828890 13f79535-47bb-0310-9956-ffa450edef68