channeldb: convert settled boolean to state

This commit is a preparation for the addition of new invoice
states. A database migration is not needed because we keep
the same field length and values.

rpcserver.go

@@ -3303,6 +3303,8 @@ func createRPCInvoice(invoice *channeldb.Invoice) (*lnrpc.Invoice, error) {
 	satAmt := invoice.Terms.Value.ToSatoshis()
 	satAmtPaid := invoice.AmtPaid.ToSatoshis()
 
+	isSettled := invoice.Terms.State == channeldb.ContractSettled
+
 	return &lnrpc.Invoice{
 		Memo:            string(invoice.Memo[:]),
 		Receipt:         invoice.Receipt[:],
@@ -3311,7 +3313,7 @@ func createRPCInvoice(invoice *channeldb.Invoice) (*lnrpc.Invoice, error) {
 		Value:           int64(satAmt),
 		CreationDate:    invoice.CreationDate.Unix(),
 		SettleDate:      settleDate,
-		Settled:         invoice.Terms.Settled,
+		Settled:         isSettled,
 		PaymentRequest:  paymentRequest,
 		DescriptionHash: descHash,
 		Expiry:          expiry,