mod_cache: Fix an out of memory condition that occurs when the

cache tries to save huge files (greater than RAM). Buckets bigger
than a tuneable threshold are split into smaller buckets before
being passed to mod_disk_cache, etc. PR 39380


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@467655 13f79535-47bb-0310-9956-ffa450edef68