core: Add ap_reuse_brigade_from_pool().

Current RETRIEVE_BRIGADE_FROM_POOL macro from "http_request.c" is turned into
a helper and used in ap_request_core_filter().

We will need it in a subsequent commit in "util_filter.c" too.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836018 13f79535-47bb-0310-9956-ffa450edef68