mod_status: Remove duplicate keys "BusyWorkers" and "IdleWorkers".

mod_status: Remove duplicate keys "BusyWorkers" and "IdleWorkers".
Resolve inconsistency between the previous two occurrences by
counting workers in state SERVER_GRACEFUL no longer as busy,
but instead in a new counter "GracefulWorkers" (or on HTML
view as "workers gracefully restarting"). Also add the graceful
counter as a new column to the existing HTML per process table
for async MPMs.
PR 63300

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1909429 13f79535-47bb-0310-9956-ffa450edef68