Add resume option to ParallelStreamingDataset

[philgzl]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #649.

• Automatic state loading and saving is now done at the beginning of iter() instead of at the end.
• Automatic state loading and saving is now controlled by a new option resume.
• Refactored ParallelStreamingDataset tests a bit.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83%. Comparing base (1c51664) to head (1360439).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #650   +/-   ##
===================================
  Coverage    83%    83%           
===================================
  Files        49     49           
  Lines      6756   6758    +2     
===================================
+ Hits       5636   5639    +3     
+ Misses     1120   1119    -1     

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[tchaton]

real nice