Multi Tenant App Support

[RPangrle]

We currently have some problematic code that might cause issues in multi tenant apps if they start from common endpoint:

         //Always ensure that the cache item has all of these set, especially in the broad token case, where the passed item
         //may have empty "resource" property:
         resultItem.resource = _resource;
         resultItem.clientId = _clientId;
         resultItem.authority = _context.authority;

Changing this behavior could have adverse effects on already existing apps in ways that we might not want to inflict on developers if we don't have to. For now we'll track this in this item and if the issue comes up we'll have to solve it somehow.