Fix assertion failure during very high load by preventing race condition

Fix assertion failure during very high load by preventing race condition
between appending to the timeout queues and adding to the pollset. We don't
add additional locking calls but only extend the present calls to include the
apr_pollset_add. Therefore this hopefully should not cause too much performance
regression.

Add some comments

Replace two AP_DEBUG_ASSERTS with better error handling

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1201146 13f79535-47bb-0310-9956-ffa450edef68