Use VK from zkApp transaction to verify proofs in it

[deepthiskumar]

When verifying transactions in the pool, we currently verify all the zkapp proofs using the VK from the best tip ledger. If a zkapp transaction has an update that sets the VK then use that to verify subsequent account updates with proofs

[bkase]

Even after @mrmr1993 's fix to also ensure we're running this logic against the mempool, there's still an edge case in the mempool which @deepthiskumar pointed out today:

User_command.to_verifiable passes a ledger to use as a fallback to find the latest verification key to depend on, however, the mempool could have a stack of transactions with earlier nonces that change the verification key which we need to rely on instead.

So we need to change this "fallback lookup" logic to (in the mempool case) lookup the latest verification key in some smarter mempool-aware mechanism before falling back to ledger.