Export Consumer.Frozen and add http introspection handler

The http handler is copied from our internal metafora runner.
Good question. For better or worse (I honestly don't know), the node name is an **etcd** Coordinator-only concept. It's not used or exposed anywhere in the core Consumer or interfaces.

A better idea might be to make the name a Consumer level concept, and the Consumer informs the Coordinator of its name via the ``Coordinator.Init()`` method.

Seems out of scope for this PR, so I created #80. Further comments welcome! I think it's a good enhancement.