Wrapper for Arrow Datasets & Dataset Pieces

[aperiodic]

This is a wrapper around PyArrow's ParquetDataset and ParquetDatasetPieces, the first part of the effort to support PyArrow's new Dataset API that was discussed in issue #613.

Since this wrapper has no functionality of its own, I didn't add unit tests specifically for the wrapper; let me know if I should.

I was unable to get the tests in petastorm/tests/test_tf_dataset.py to complete when running them inside the provided docker container, even when I left them to run for over 24 hours. Given that this is only a wrapper, and the code paths where the wrapper is used are covered by other tests, I think it's unlikely that tests in that file would fail when all other tests pass, but I'll keep an eye on the CI results. If they fail, then don't bother reviewing this until I fix them up (which may take several rounds of pushing new commits, as I can only run those tests in CI and not locally).

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[codecov]

Codecov Report

❌ Patch coverage is 94.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.38%. Comparing base (d32709d) to head (819a127).
⚠️ Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
petastorm/pyarrow_helpers/dataset_wrapper.py	96.42%	0 Missing and 2 partials ⚠️
petastorm/etl/metadata_util.py	0.00%	1 Missing ⚠️
petastorm/etl/rowgroup_indexing.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #754      +/-   ##
==========================================
+ Coverage   86.27%   86.38%   +0.11%     
==========================================
  Files          85       86       +1     
  Lines        5084     5141      +57     
  Branches      787      793       +6     
==========================================
+ Hits         4386     4441      +55     
  Misses        559      559              
- Partials      139      141       +2     

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[selitvin]

LGTM! Do you think we can keep it in a PR for now until we have a followup that builds on this PR with the actual new version of a dataset being used?

[selitvin]

Please add a comment explaining why do we need the wrapper.

[westfly]

good feature