R1:07b60b201625

Require valid class for certain config settings

Summary:
It is now possible to set config setting requiring class of certain implementation to something completely else.
The consequence is that your Phabricator may stop working after update because you didn't implement some new method.

This diff validates the class upon usage.
It throws exception which is better than fatal thrown currently after calling undefined method.

Better solution would be to validate classes when setting the config but it would…