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.

invoices/mock.go

@@ -0,0 +1,78 @@
+package invoices
+
+import (
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/stretchr/testify/mock"
+)
+
+type MockInvoiceDB struct {
+	mock.Mock
+}
+
+func NewInvoicesDBMock() *MockInvoiceDB {
+	return &MockInvoiceDB{}
+}
+
+func (m *MockInvoiceDB) AddInvoice(invoice *Invoice,
+	paymentHash lntypes.Hash) (uint64, error) {
+
+	args := m.Called(invoice, paymentHash)
+
+	addIndex, _ := args.Get(0).(uint64)
+
+	// NOTE: this is a side effect of the AddInvoice method.
+	invoice.AddIndex = addIndex
+
+	return addIndex, args.Error(1)
+}
+
+func (m *MockInvoiceDB) InvoicesAddedSince(idx uint64) ([]Invoice, error) {
+	args := m.Called(idx)
+	invoices, _ := args.Get(0).([]Invoice)
+
+	return invoices, args.Error(1)
+}
+
+func (m *MockInvoiceDB) InvoicesSettledSince(idx uint64) ([]Invoice, error) {
+	args := m.Called(idx)
+	invoices, _ := args.Get(0).([]Invoice)
+
+	return invoices, args.Error(1)
+}
+
+func (m *MockInvoiceDB) LookupInvoice(ref InvoiceRef) (Invoice, error) {
+	args := m.Called(ref)
+	invoice, _ := args.Get(0).(Invoice)
+
+	return invoice, args.Error(1)
+}
+
+func (m *MockInvoiceDB) ScanInvoices(scanFunc InvScanFunc,
+	reset func()) error {
+
+	args := m.Called(scanFunc, reset)
+
+	return args.Error(0)
+}
+
+func (m *MockInvoiceDB) QueryInvoices(q InvoiceQuery) (InvoiceSlice, error) {
+	args := m.Called(q)
+	invoiceSlice, _ := args.Get(0).(InvoiceSlice)
+
+	return invoiceSlice, args.Error(1)
+}
+
+func (m *MockInvoiceDB) UpdateInvoice(ref InvoiceRef, setIDHint *SetID,
+	callback InvoiceUpdateCallback) (*Invoice, error) {
+
+	args := m.Called(ref, setIDHint, callback)
+	invoice, _ := args.Get(0).(*Invoice)
+
+	return invoice, args.Error(1)
+}
+
+func (m *MockInvoiceDB) DeleteInvoice(invoices []InvoiceDeleteRef) error {
+	args := m.Called(invoices)
+
+	return args.Error(0)
+}