Bug 1256608

+++ This bug was initially created as a clone of Bug #1256607 +++

### Description of problem:

If all threads in a httpd child worker process are saturated with long requests, then all connections in the pool are exhausted, likely leaving none available for additional pings, which then fail with errors like:

    [error] (70007)The timeout specified has expired: proxy: ajp: failed to acquire connection for ...


### Steps to Reproduce:

1. Deploy a web application on EAP 6, which just do Thread.sleep() for a while
   (for example, 10-15 seconds. just longer than lbstatus_recalc_time, 5 seconds)

    ~~~
    <%
        long ms = 15000L;
        System.out.println("going to sleep... : " + ms + " ms");
        Thread.sleep(ms);
        System.out.println("done!");
    %>
    ~~~

2. Repeat sending concurrent requests to the web app via mod_cluster by apache bench
   (just exactly same concurrent number to MaxClients setting, or more than MaxClients)

   When you set MaxClients to 30, then execute the following:

    ~~~
    ab -c 30 -n 300 http://localhost/example/sleep.jsp
    ~~~


### Actual results:

You will see the "failed to acquire connection for ..." error message when status update is executed in background.


### Expected results:

No error message.


### Additional info:

https://issues.jboss.org/browse/MODCLUSTER-466