core: core output filter to cleanup empty/meta buckets following data, ASAP.

core: core output filter to cleanup empty/meta buckets following data, ASAP.

* server/core_filters.c (delete_meta_bucket):
  Add delete_meta_bucket() helper to set r->flush on EOR.

* server/core_filters.c (send_brigade_nonblocking):
  Use delete_meta_bucket() to factorize code.

* server/core_filters.c (writev_nonblocking):
  Use delete_meta_bucket() to factorize code.
  Cleanup trailing empty/meta buckets immediately after data have been sent to
  get more accurate EOR timestamp (otherwise trailing buckets could be deleted
  only after the next data or with a final send_brigade_nonblocking() call in
  the output filter).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1892452 13f79535-47bb-0310-9956-ffa450edef68