The BlindedPathConfig struct is nice for invoice creation but when we
use the Invoice message for viewing an invoice, it would be nicer to see
an "is_blinded" field.
The only way to know if an invoice is AMP, Keysend, etc is to look at
its shape/characteristics. This commit adds a couple of helper functions
to encapsulate the logic of these checks.
If all these types cannot intersect: an invoice cannot be AMP and
Keysend or Keysend and Bolt12, etc it could be useful to add an extra
field to store this information instead of relying on checking how the
invoice looks like.
Now that we have the new package `lnd/channeldb/models` we can invert the
depenency between `channeldb` and `invoices`.
- Move all the invoice related types and errors to the
`invoices` package.
- Ensure that all the packages dealing with invoices use the types and
interfaces defined in the `invoices` package.
- Implement the InvoiceDB interface (defined in `lnd/invoices`) in
channeldb.
- Add new mock for InterfaceDB.
- `InvoiceRegistery` tests are now in its own subpacakge (they need to
import both invoices & channeldb). This is temporary until we can
decouple them.
