apr_pollset_remove: use a separate aiocb pointing to the original for cancelling async i/o.  this fixes a leak
in USS for every keepalive timeout combined with CPU growth on the httpd listener thread 


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1434976 13f79535-47bb-0310-9956-ffa450edef68