Merge r1828890, r1832500 from trunk:

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.


mod_proxy_balancer: follow up to r1828890: indentation and 80 col.


Submitted by: jhriggs, ylavic
Reviewed by: jhriggs, jim, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1832609 13f79535-47bb-0310-9956-ffa450edef68