mod_logio modification: count bytes-sent after the writev or sendfile

mod_logio modification: count bytes-sent after the writev or sendfile
call in the core_output_filter, in order to get a more accurate count
of the total bytes transmitted in cases where the client terminates
the connection before the entire response is sent.

Note: This works by adding a flush bucket to each response when
mod_logio is used; the side-effect is that pipelined responses
get broken up into separate network writes per request (but there's
no impact on pipelining when mod_logio is not enabled).

Submitted by:	Bojan Smojver <bojan@rexursive.com>
Reviewed by:	Brian Pane

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97300 13f79535-47bb-0310-9956-ffa450edef68