htlcswitch: hodl invoice

This commit modifies the invoice registry to handle invoices for which
the preimage is not known yet (hodl invoices). In that case, the
resolution channel passed in from links and resolvers is stored until we
either learn the preimage or want to cancel the htlc.

lnrpc/invoicesrpc/utils.go

@@ -60,6 +60,8 @@ func CreateRPCInvoice(invoice *channeldb.Invoice,
 		state = lnrpc.Invoice_SETTLED
 	case channeldb.ContractCanceled:
 		state = lnrpc.Invoice_CANCELED
+	case channeldb.ContractAccepted:
+		state = lnrpc.Invoice_ACCEPTED
 	default:
 		return nil, fmt.Errorf("unknown invoice state %v",
 			invoice.Terms.State)