output filter optimizations:

- avoid creating a new bucket brigade for each EOS or FLUSH bucket
- do not pass FLUSH bucket since BIO_bucket_flush() already does so
  if there is any data to flush.
- do not call apr_brigade_destroy(bb); since we will have either
  emptied it or passed it, in which case it will be destroyed further
  down the chain.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92885 13f79535-47bb-0310-9956-ffa450edef68