Implement local pay method

[mkurapov]

Context

To actually make the local payments happen, we will need to implement the pay method of the new local payment method handling service.

This method will call the accountingService.createTransfer function which will make a transfer between the outgoing payment (the source account) and the incoming payment (destination account) using the amounts from the original quote. This can be a instantly-posted transfer, meaning, we don't need to create a pending transfer with a timeout, we can just make the accounting transfer instantly.