channeldb: store optional invoice fields as variable length byte arrays

This commit modifies the on-disk storage of invoices to stop the
optional fields (memo+receipt) on-disk as variable length byte arrays.
This change saves space as the optional fields now only take up as much
space as is strictly needed, rather than always being padded out to max
size (1KB).

rpcserver.go

@@ -613,12 +613,12 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 
 	i := &channeldb.Invoice{
 		CreationDate: time.Now(),
+		Memo:         []byte(invoice.Memo),
+		Receipt:      invoice.Receipt,
 		Terms: channeldb.ContractTerm{
 			Value: btcutil.Amount(invoice.Value),
 		},
 	}
-	copy(i.Memo[:], invoice.Memo)
-	copy(i.Receipt[:], invoice.Receipt)
 	copy(i.Terms.PaymentPreimage[:], preImage)
 
 	if err := r.server.invoices.AddInvoice(i); err != nil {