multi: break invoice depenency on channeldb

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.

htlcswitch/mock.go

@@ -980,7 +980,7 @@ func newMockRegistry(minDelta uint32) *mockInvoiceRegistry {
 }
 
 func (i *mockInvoiceRegistry) LookupInvoice(rHash lntypes.Hash) (
-	channeldb.Invoice, error) {
+	invoices.Invoice, error) {
 
 	return i.registry.LookupInvoice(rHash)
 }
@@ -1014,7 +1014,7 @@ func (i *mockInvoiceRegistry) CancelInvoice(payHash lntypes.Hash) error {
 	return i.registry.CancelInvoice(payHash)
 }
 
-func (i *mockInvoiceRegistry) AddInvoice(invoice channeldb.Invoice,
+func (i *mockInvoiceRegistry) AddInvoice(invoice invoices.Invoice,
 	paymentHash lntypes.Hash) error {
 
 	_, err := i.registry.AddInvoice(&invoice, paymentHash)