Use a "Proof Chain" for witness proofs

[brianorwhatever]

As shown here I think it might be a good idea to include the previousProof property to a witness proof. This will show that the witnesses are signing on top of an already signed proof by the controller. It will also give us a simple way to know which proofs to remove from the log entry when creating a new one.

[swcurran]

Nice! I like that. I’ll see about updating the spec. to include that.