Processor modularity

[kayabaNerve]

Right now, the Scheduler expects a UTXO interface. I'm not saying we can't cram an account-based chain such as ETH into it, yet we likely want to have networks specify their scheduler.

Ideally, we could also modularize out fuzzy matching (outputs + plan ID match, as used in Monero) vs strict matching (we know a TX ID before we sign the TX). The latter would let us chain TXs, instead of waiting for on-chain appearance. This would massively increase the complexity of #284 though, as it'd be n-depth RBF instead of 1-deep RBF.

[kayabaNerve]

Partially done during the Ethereum integration and superseded by #569.