在where中用Date类型去比较大小，发现会出错，查询出现了之前的数据，导致数据重复
timestamp和lastId是Date类型
msgs = try db.getObjects(fromTable: "Message",where: MessageDB.Properties.conversationId == conversationId && MessageDB.Properties.timestamp < lastId!,orderBy: [MessageDB.Properties.timestamp.order(.descending)],limit: 5)