code: void setLastMessage(AVIMMessage lastMessage) {
      if (null != lastMessage) {
        if (null == this.lastMessage) {
          this.lastMessage = lastMessage;
        } else {
          if(this.lastMessage.getTimestamp() <= lastMessage.getTimestamp()) {
            this.lastMessage = lastMessage;
          }
        }
      }
    }
docs: lastMessage 的来源：
  1. sqlite conversation 表中的 lastMessage，conversation query 后存储下来的
  2. sqlite message 表中存储的最新的消息
  3. 实时通讯推送过来的消息也要及时更新 lastMessage
  @param lastMessage