Use latest replication task time as sync shard time (#3722)

Author: yux0

service/history/replication/ack_manager.go

@@ -247,12 +247,16 @@ func (p *ackMgrImpl) GetTasks(
 	p.metricsHandler.Histogram(metrics.ReplicationTasksReturned.GetMetricName(), metrics.ReplicationTasksReturned.GetMetricUnit()).
 		Record(int64(len(replicationTasks)))
 
+	replicationEventTime := timestamp.TimePtr(p.shard.GetTimeSource().Now())
+	if len(replicationTasks) > 0 {
+		replicationEventTime = replicationTasks[len(replicationTasks)-1].GetVisibilityTime()
+	}
 	return &replicationspb.ReplicationMessages{
 		ReplicationTasks:       replicationTasks,
 		HasMore:                lastTaskID < maxTaskID,
 		LastRetrievedMessageId: lastTaskID,
 		SyncShardStatus: &replicationspb.SyncShardStatus{
-			StatusTime: timestamp.TimePtr(p.shard.GetTimeSource().Now()),
+			StatusTime: replicationEventTime,
 		},
 	}, nil
 }