core: Restore "core_output_filter: writing data to the network", but at TRACE1.

trunk patch: http://svn.apache.org/r1501294

Submitted by: covener
Reviewed by: jim, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523262 13f79535-47bb-0310-9956-ffa450edef68