core_output_filter improvements:

- prevent more than 5 pipelined responses in the output brigade, to avoid
  excessive FD usage and related DoS attacks.
- if non_file_bytes_in_brigade >= THRESHOLD_MAX_BUFFER, don't send the 
  entire brigade non-blocking, but only up to the bucket that brought us
  over THRESHOLD_MAX_BUFFER. This should allow better use of async write
  completion.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@956204 13f79535-47bb-0310-9956-ffa450edef68