multi: make decayed log DB remote compatible

Even though the sphinx router's persistent replay log is not crucial in
the operation of lnd as its state can be re-created by creating a new
brontide connection, we want to make lnd fully stateless and therefore
have the option of not storing any state on disk.

lnd.go

@@ -1616,6 +1616,7 @@ type databaseInstances struct {
 	chanStateDB  *channeldb.DB
 	heightHintDB kvdb.Backend
 	macaroonDB   kvdb.Backend
+	decayedLogDB kvdb.Backend
 }
 
 // initializeDatabases extracts the current databases that we'll use for normal
@@ -1649,6 +1650,7 @@ func initializeDatabases(ctx context.Context,
 	dbs := &databaseInstances{
 		heightHintDB: databaseBackends.HeightHintDB,
 		macaroonDB:   databaseBackends.MacaroonDB,
+		decayedLogDB: databaseBackends.DecayedLogDB,
 	}
 	cleanUp := func() {
 		// We can just close the returned close functions directly. Even