Introduce an socache iterator 'userctx' for the user to track the

context of their own callback.

Breaks API, but we probably wanted an mmn reset between .6 and .7
betas for unintentional/unnoticed ABI breaks.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@958157 13f79535-47bb-0310-9956-ffa450edef68