Delete the data file if it has no valid data

Currently, when we execute "delete from root.sg.* where time < T", we write modifications besides the deleted TsFile.

It's better to delete the file that does not have valid data.

首先跟踪 `delete from root.sg.* where time < T` 如何处理受影响的 TsFile 并写入修改记录。确定系统如何识别文件不包含有效数据，然后验证此类文件会被删除，而不是带着修改记录保留。

由索引模型根据 Issue 内容生成。