New version of ap_core_output_filter...

New version of ap_core_output_filter...
The big change is that it now does nonblocking writes when possible.  The
goal of this redesign is to more cleanly support asynchronous write completion
in MPMs such as Event or Leader.  However, the nonblocking writes may also
help in Worker and Prefork by allowing more overlapping of handler and
output filter processing with network writes.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/async-dev@291377 13f79535-47bb-0310-9956-ffa450edef68