skiplist: Add skiplist capability to httpd

2.4.x patch: http://people.apache.org/~jim/patches/httpd-2.4-skiplist.patch

Submitted by: jim
Reviewed by: druggeri, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523254 13f79535-47bb-0310-9956-ffa450edef68