Add Metering Feature to allow nodes to pre-pay fees
We should allow nodes and notaries to pre-pay Metering fees upfront so that they can reduce the their overall transaction costs and take advantage of pre pay offers. To do this, we introduce a new metering state ‘PRE-PAID’ on the metering invoice which is initiated by the node and sent to the metering notary with the accompanying payment. The metering notary will update the Metering T&C for the party with the new free credit limit OR It will update the PRE-PAID invoice until all the credit is used.
Note: the latter would require notarisation which sort of defeats the point - but would work for bulk invoices/payments The other alternative is offset the pre-paid against unpaid, and then only reconcile when an invoice needs to be raised.