mod_proxy: Fix a race condition that caused a failed worker to be retried

before the retry period is over

Backports: r1664709, r1697323
Submitted by: rpluem
Reviewed by: wrowe, ylavic



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1750844 13f79535-47bb-0310-9956-ffa450edef68