invoiecs: fix value conversion bug in AddHodlInvoice

Before this commit, callers wouldn't be able to use an msat amount as
input to the hold invoice RPC call. Instead, the wrapper logic would
always convert to msat first, causing a loss in precision. For most
cases, sat amounts are fine, but some use cases require higher
precision.

We fix this by keeping the input value in mSat (as typed), while using
the `ValueMsat` field as `lnd` would expect.

invoices_client.go

@@ -227,7 +227,7 @@ func (s *invoicesClient) AddHoldInvoice(ctx context.Context,
 	rpcIn := &invoicesrpc.AddHoldInvoiceRequest{
 		Memo:       in.Memo,
 		Hash:       in.Hash[:],
-		Value:      int64(in.Value.ToSatoshis()),
+		ValueMsat:  int64(in.Value),
 		Expiry:     in.Expiry,
 		CltvExpiry: in.CltvExpiry,
 		Private:    true,