core_output_filter: re-instate the deferred_write pool patch so we don't

leak fd's until the end of a keepalive connection.

Thanks to:
  Jeff Trawick for the original concept
  Sander Striker for the mmap ring idea
  Cliff Woolley for implementing the above change


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97824 13f79535-47bb-0310-9956-ffa450edef68