mpm_event: don't kill keepalive connections on connections_above_limit().

mpm_event: don't kill keepalive connections on connections_above_limit().

Before r1819855 (backported to 2.4.30), mpm_event killed keepalive connections
only when workers were exhausted, while this commit set workers_were_busy for
connections_above_limit().

Restore prior to r1819855 behaviour, and since ap_queue_info_num_idlers() is
now part of connections_above_limit(), let's update workers_were_busy there
only when necessary.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1883096 13f79535-47bb-0310-9956-ffa450edef68