fix: miss to save ts for iterator checkpoint(#2412)

[MrPresent-Han]

related: #2412

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@d875186). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pymilvus/orm/iterator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3147   +/-   ##
=========================================
  Coverage          ?   46.10%           
=========================================
  Files             ?       66           
  Lines             ?    14215           
  Branches          ?        0           
=========================================
  Hits              ?     6554           
  Misses            ?     7661           
  Partials          ?        0           

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