Merge r1294356 from trunk:

Take care not to call ap_start_lingering_close from the listener thread,
because it may block when flushing data to the client.

From the listener thread, do a lingering close without flushing. This is
OK because we only do this if there has been an error during write
completion or if our send buffers are empty because we are in keep-alive.

PR: 52229

Submitted by: sf
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1364613 13f79535-47bb-0310-9956-ffa450edef68