util_filter: Axe conn_rec->empty brigade.

Since it's internal util_filter use, we shouldn't expose it in conn_rec and
can replace it with a pooled brigade provided by ap_reuse_brigade_from_pool().


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836020 13f79535-47bb-0310-9956-ffa450edef68