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/errors.go

@@ -0,0 +1,108 @@
+package invoices
+
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	// ErrInvoiceAlreadySettled is returned when the invoice is already
+	// settled.
+	ErrInvoiceAlreadySettled = errors.New("invoice already settled")
+
+	// ErrInvoiceAlreadyCanceled is returned when the invoice is already
+	// canceled.
+	ErrInvoiceAlreadyCanceled = errors.New("invoice already canceled")
+
+	// ErrInvoiceAlreadyAccepted is returned when the invoice is already
+	// accepted.
+	ErrInvoiceAlreadyAccepted = errors.New("invoice already accepted")
+
+	// ErrInvoiceStillOpen is returned when the invoice is still open.
+	ErrInvoiceStillOpen = errors.New("invoice still open")
+
+	// ErrInvoiceCannotOpen is returned when an attempt is made to move an
+	// invoice to the open state.
+	ErrInvoiceCannotOpen = errors.New("cannot move invoice to open")
+
+	// ErrInvoiceCannotAccept is returned when an attempt is made to accept
+	// an invoice while the invoice is not in the open state.
+	ErrInvoiceCannotAccept = errors.New("cannot accept invoice")
+
+	// ErrInvoicePreimageMismatch is returned when the preimage doesn't
+	// match the invoice hash.
+	ErrInvoicePreimageMismatch = errors.New("preimage does not match")
+
+	// ErrHTLCPreimageMissing is returned when trying to accept/settle an
+	// AMP HTLC but the HTLC-level preimage has not been set.
+	ErrHTLCPreimageMissing = errors.New("AMP htlc missing preimage")
+
+	// ErrHTLCPreimageMismatch is returned when trying to accept/settle an
+	// AMP HTLC but the HTLC-level preimage does not satisfying the
+	// HTLC-level payment hash.
+	ErrHTLCPreimageMismatch = errors.New("htlc preimage mismatch")
+
+	// ErrHTLCAlreadySettled is returned when trying to settle an invoice
+	// but HTLC already exists in the settled state.
+	ErrHTLCAlreadySettled = errors.New("htlc already settled")
+
+	// ErrInvoiceHasHtlcs is returned when attempting to insert an invoice
+	// that already has HTLCs.
+	ErrInvoiceHasHtlcs = errors.New("cannot add invoice with htlcs")
+
+	// ErrEmptyHTLCSet is returned when attempting to accept or settle and
+	// HTLC set that has no HTLCs.
+	ErrEmptyHTLCSet = errors.New("cannot settle/accept empty HTLC set")
+
+	// ErrUnexpectedInvoicePreimage is returned when an invoice-level
+	// preimage is provided when trying to settle an invoice that shouldn't
+	// have one, e.g. an AMP invoice.
+	ErrUnexpectedInvoicePreimage = errors.New(
+		"unexpected invoice preimage provided on settle",
+	)
+
+	// ErrHTLCPreimageAlreadyExists is returned when trying to set an
+	// htlc-level preimage but one is already known.
+	ErrHTLCPreimageAlreadyExists = errors.New(
+		"htlc-level preimage already exists",
+	)
+
+	// ErrInvoiceNotFound is returned when a targeted invoice can't be
+	// found.
+	ErrInvoiceNotFound = errors.New("unable to locate invoice")
+
+	// ErrNoInvoicesCreated is returned when we don't have invoices in
+	// our database to return.
+	ErrNoInvoicesCreated = errors.New("there are no existing invoices")
+
+	// ErrDuplicateInvoice is returned when an invoice with the target
+	// payment hash already exists.
+	ErrDuplicateInvoice = errors.New(
+		"invoice with payment hash already exists",
+	)
+
+	// ErrDuplicatePayAddr is returned when an invoice with the target
+	// payment addr already exists.
+	ErrDuplicatePayAddr = errors.New(
+		"invoice with payemnt addr already exists",
+	)
+
+	// ErrInvRefEquivocation is returned when an InvoiceRef targets
+	// multiple, distinct invoices.
+	ErrInvRefEquivocation = errors.New("inv ref matches multiple invoices")
+
+	// ErrNoPaymentsCreated is returned when bucket of payments hasn't been
+	// created.
+	ErrNoPaymentsCreated = errors.New("there are no existing payments")
+)
+
+// ErrDuplicateSetID is an error returned when attempting to adding an AMP HTLC
+// to an invoice, but another invoice is already indexed by the same set id.
+type ErrDuplicateSetID struct {
+	SetID [32]byte
+}
+
+// Error returns a human-readable description of ErrDuplicateSetID.
+func (e ErrDuplicateSetID) Error() string {
+	return fmt.Sprintf("invoice with set_id=%x already exists", e.SetID)
+}