Merge r1534321, r1550061 from trunk:

* modules/proxy/mod_proxy_http.c (ap_proxy_http_request): Use the same
  brigade lifetime for the header brigade as the other brigades.
  POSSIBLE (but as yet unconfirmed) fix for crashes seen with threaded
  servers, e.g. PR 50335; appears correct or at least not harmful.

PR: 50335


Cleanup the bb brigade, because buckets inserted to it can be created from
scpool and this pool can be freed before this brigade.
POSSIBLE (but as yet unconfirmed) fix for crashes seen with threaded servers,
e.g. PR 50335.

Reviewed by: rpluem, jorton, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1573067 13f79535-47bb-0310-9956-ffa450edef68