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/interfaces.go

@@ -18,7 +18,7 @@ import (
 type InvoiceDatabase interface {
 	// LookupInvoice attempts to look up an invoice according to its 32
 	// byte payment hash.
-	LookupInvoice(lntypes.Hash) (channeldb.Invoice, error)
+	LookupInvoice(lntypes.Hash) (invoices.Invoice, error)
 
 	// NotifyExitHopHtlc attempts to mark an invoice as settled. If the
 	// invoice is a debug invoice, then this method is a noop as debug