improvement: avoid doubly storing vk for Proofs

[distractedm1nd]

After the Account refactor, we now pass the vk in the Transaction. As this is also stored in the Account, it gets serialized twice inside of InsertProof/UpdateProof.

We could use key_idx like we did previously, but this should be done in a way that has a nice UX.

I am leaving it as an additional issue because this is an optimization that could damage UX and readability if we don't do it in a nice way.

Originally posted by @distractedm1nd in #180 (comment)