Proof-of-concept implementation of lease renewal and price calculation using tahoe's iteration.

Created by: tomprince

This is completely untested. It isn't obvious if the code can use the existing fakes, and I don't have setup to test live.

Also, the code is really messy, though it'd be straightforward to refactor.

(Incidentally, I think I may know how to hook into the operations stuff, if we want to present the price result that way, rather than waiting)