lnrpc: set a zero value endorsement signal on sender outgoing htlc

Before we have sufficient signaling in the network to relay this
signal, set a zero value experimental endorsement value on the sender's
outgoing htlc. Once the network is relaying this signal and a flag day
has been set, we'll be able to set a non-zero value here.

rpcserver.go

@@ -44,6 +44,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb/graphsession"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/channelnotifier"
+	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/feature"
@@ -758,6 +759,11 @@ func (r *rpcServer) addDeps(s *server, macService *macaroons.Service,
 
 			return nil
 		},
+		ShouldSetExpEndorsement: func() bool {
+			return clock.NewDefaultClock().Now().Before(
+				EndorsementExperimentEnd,
+			)
+		},
 	}
 
 	genInvoiceFeatures := func() *lnwire.FeatureVector {