Clean up the http filter a lot. Instead of sending a bucket per header,

we now create a single bucket that has all of the headers and send that.
One known issue is that this limits the headers to 8k, but it is a minor
patch to fix that.

This also stops us from exporting ap_send_header_field.  This function no
longer makes sense to export.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86780 13f79535-47bb-0310-9956-ffa450edef68