multi: add preimage push to loop out after sweep attempt

Once we have revealed our preimage to the world with a sweep attempt,
we can safely push our preimage to the server to speed up on chain
claim.

Rather than rely on the server, we use the state of our invoice in lnd
to determine whether we should continue trying to push the preimage to
the server.

server_mock_test.go

@@ -36,6 +36,9 @@ type serverMock struct {
 
 	swapInvoice string
 	swapHash    lntypes.Hash
+
+	// preimagePush is a channel that preimage pushes are sent into.
+	preimagePush chan lntypes.Preimage
 }
 
 var _ swapServerClient = (*serverMock)(nil)
@@ -49,6 +52,8 @@ func newServerMock() *serverMock {
 		prepayInvoiceAmt: 100,
 
 		height: 600,
+
+		preimagePush: make(chan lntypes.Preimage),
 	}
 }
 
@@ -151,6 +156,15 @@ func (s *serverMock) NewLoopInSwap(ctx context.Context,
 	return resp, nil
 }
 
+func (s *serverMock) PushLoopOutPreimage(_ context.Context,
+	preimage lntypes.Preimage) error {
+
+	// Push the preimage into the mock's preimage channel.
+	s.preimagePush <- preimage
+
+	return nil
+}
+
 func (s *serverMock) GetLoopInTerms(ctx context.Context) (
 	*LoopInTerms, error) {