Add deferred write pool to core_output_filter to clean up file descriptors as

soon as the corresponding response has been sent and not only after the
connection has been closed.

This change does not deal with pipelined requests, yet.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@956202 13f79535-47bb-0310-9956-ffa450edef68