mod_cache_socache: New cache implementation backed by mod_socache

that replaces mod_mem_cache removed from httpd v2.2.

Add backported files, missed by the backport process in r1477652.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1477708 13f79535-47bb-0310-9956-ffa450edef68