Fix: The current preimage of a invoice's lightning payment method should be available via API

[NicolasDorier]

We are serializing payment method details in two places: Payments blob and inside the Invoices blob.

Sadly, we aren't using the NBitcoin's serializer for the payment method detail in the Invoices blob. resulting in the unuseful

{
  "PaymentHash": { "Size": 32 }
}

Being saved in the database. Notice the casing is wrong.

We will really need to refactor everything concerning payment methods and payment methods details very soon.

[Kukks]

Damn so we have not had any of this data properly persisted?

[NicolasDorier]

@Kukks we actually had the data, but at the payment level (we receive preimage when we receive the payment), not invoice level.
I am working on another PR that may be better. That would use proper serialized, and fix the JSON in string at invoice level.

[NicolasDorier]

Giving up the other PR, need complete refactor of the PaymentType stuff, it's too big. Will probably just push this PR.