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.
Add a new subpackage to `lnd/channeldb` to hold some of the types that
are used in the package itself and in other packages that should not
depend on `channeldb`.
This commit splits the resolution result enum into results divided
by outcome (settled, failed or accepted). This allows us to more
strictly control which resolution results can be used with which
HtlcResolution structs, to prevent the combination of a settle
resolution result with a failure resolution result, for example.
This commit repalces the htlcResolution struct with an interface.
This interface is implemeted by failure, settle and accept resolution
structs. Only settles and fails are exported because the existing
code that handles htlc resolutions uses a nil resolution to indicate
that a htlc was accepted. The accept resolution is used internally
to report on the resolution result of the accepted htlc, but a nil
resolution is surfaced. Further refactoring of all the functions
that call NotifyExitHopHtlc to handle a htlc accept case (rather than
having a nil check) is required.
