blockchain: facility for tracking utxo proofs

[oleganza]

It's turning out to be clunky to handle utxo proofs in and out of various blockchain state machine procedures. Especially, since it's unclear in which cases user cares or not about the proofs. Would be good to abstract that away with a "dependency-injection" API that lets the core algorithms request and store utxo proofs, but not worry about how exactly they are represented, where they come from and whether the user even wants to update any proofs.

TBD: write this up in more detail

[oleganza]

Replaced by #426