backwards-compat for v3

this should be fairly easy, since we're already allowing to bootstrap from an eip file. to be fully compatible with existing clients, we need to serve all the rest of stuff that bonafide contemplated.

It'd be good if all the additions are self-contained, and optionally enabled with a flag, so that they can be neatly deprecated when the time comes.

(I would make v3 compat false by default for new deployments).