Augment VoucherManager to "keep a tab" on individual counterparties

[NiloCK]

Go-nitro seeks to facilitate exchange of value, where one side provides some crypto asset and the other side provides some service.

Via the voucher manager, go-nitro processes and stores all relevant information for the crypto asset side of this exchange, but there is currently no mechanism to track the provision of service. This makes things a little awkward on both sides of the exchange:

• service provider has to decide when to provide further service, or to cut off service provision
• a payer has to decide when to provide or cut off further payments