Fix version vector validation when running migration (#1057)

This fixes a validation error that occurs during migration when version vectors
contain multiple actors. After the 0.5.3 update, version vectors were allowed
to have multiple actors, but the validation process incorrectly returned an
error when encountering more than one actor in a version vector.

migrations/v0.5.3/add-version-vector.go

@@ -50,16 +50,12 @@ func validateAddVersionVector(ctx context.Context, db *mongo.Client, databaseNam
 		}
 
 		versionVector := info.VersionVector
-		actors, err := versionVector.Keys()
+		actorID, err := info.ActorID.ToActorID()
 		if err != nil {
 			return err
 		}
 
-		if len(actors) > 1 {
-			return fmt.Errorf("found %d actor in version vector", len(actors))
-		}
-
-		if versionVector.VersionOf(actors[0]) != info.Lamport {
+		if versionVector.VersionOf(actorID) != info.Lamport {
 			return fmt.Errorf("wrong lamport in version vector")
 		}