Update Migration.php

[lieshanhe]

dynamic insert timestamp in the csv .dat file.

Hello!

• Type: new feature

In raising this pull request, I confirm the following (please check boxes):

• [x] I have read and understood the Contributing Guidelines
• [x] I have checked that another pull request for this purpose does not exist
• [x] I wrote some tests for this PR

Small description of change: make it's possible to add timestamp in the .dat file for batchInsert() function.

Thanks