core: follow up to r1836237: whitelist in-memory buckets.

As Eric noted, this is safer w.r.t. third party buckets which could be
uncought by the previous backlist and get out of memory limits.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836258 13f79535-47bb-0310-9956-ffa450edef68