mpm_event: wakeup the listener to re-enable listening sockets.

When listening sockets are disabled (too many connections) and the number of
workers / active connections comes back below the limit, we need to wake up
the listener to re-enable them.

Add a new connections_above_limit() helper to determine when this applies.


Follow up to r1819855: CHANGES entry.


Submitted by: ylavic
Reviewed by: ylavic, jim, icing


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1823643 13f79535-47bb-0310-9956-ffa450edef68