Tsfiles in sequence space may be overlap with each other due to LastFlushTime bug

This issue may lead to overlapped TsFile in sequence space.

For example,

When recover the last flush time map from two sequence TsFiles.

• TsFile A only contains device1 with end time = 10000
• TsFile B only contains device2 with end time = 9999

And the resources of these two TsFiles have been downgrade to FileTimeIndex.

The previous code will use TsFile B with end time = 9999 to recover the last flush time of device1, which would cause sequence files overlapped.

It is due to the recover step bug during the maintenance of LastFlushTime