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.

htlcswitch/mock.go

@@ -720,11 +720,11 @@ func (i *mockInvoiceRegistry) SettleInvoice(rhash chainhash.Hash,
 		return fmt.Errorf("can't find mock invoice: %x", rhash[:])
 	}
 
-	if invoice.Terms.Settled {
+	if invoice.Terms.State == channeldb.ContractSettled {
 		return nil
 	}
 
-	invoice.Terms.Settled = true
+	invoice.Terms.State = channeldb.ContractSettled
 	invoice.AmtPaid = amt
 	i.invoices[rhash] = invoice