mod_proxy_balancer: Add failontimeout parameter. Timeout will put worker

in error state if an IO timeout is detected.

Backports: r1465839
Submitted by: druggeri
Reviewed by: druggeri, wrowe, rjung



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497423 13f79535-47bb-0310-9956-ffa450edef68