Create "payment method handler" service, that abstracts away quoting & paying

[mkurapov]

In order to be able to support multiple payment methods, we need a service that knows how to do quoting and transfers without having the callers of the service (QuoteService, OutgoingPaymentService) know exactly about the details of how quoting and paying are done for different payment methods.

Tasks

• Introduce "payment method handler" service that abstracts away quoting and paying
• Have quoteService.createQuote call the "payment manager" service "getQuote" method
• Move ILP quoting & transfers into own service
• Have outgoingPaymentService call the "payment manager" service "pay" method