payroll: support multiple transactions in a single signing request

[RaghavSood]

Presently, the payroll plugin creates one transaction request per-recipient.

We should migrate to the messages array flow of a regular keysign request and support verification and signing of a bundle of transactions

Each transaction must still be tracked and recorded separately (as an individual transfer may fail), but it will simplify the proposal and verification logic substantially if we can bundle them properly.

At this stage, we should not retry and failed or errored transactions

[RaghavSood]

Maybe - maybe some constraints when we actually try to implement, @johnnyluo will investigate

[RaghavSood]

This is done in #100