ARROW-6558: [C++] Refactor Iterator to type erased handle

[bkietz]

To resolve linkage issues with MSVC and the current abstract base class Iterator

@pitrou

[pitrou]

@kszucs Do you think the Ursabot build failures here are related?

[pitrou]

Thanks a lot for doing this. Here's a bunch of comments.

[pitrou]

Can you comment a bit on the implementation? So that the person reading this for the first time doesn't scratch their head.

[bkietz]

Will do

[pitrou]

Ah, sorry, I wasn't clear. I meant comment on the general design (why we're using type erasure) rather than details of member variables - these can stay of course :-)

[pitrou]

It can go in another PR though :-)

[bkietz]

I'll add comments about this in #5221

[codecov-io]

Codecov Report

Merging #5391 into master will increase coverage by 0.57%.
The diff coverage is 98.92%.

@@            Coverage Diff             @@
##           master    #5391      +/-   ##
==========================================
+ Coverage   88.58%   89.16%   +0.57%     
==========================================
  Files         950      758     -192     
  Lines      126207   110762   -15445     
  Branches     1495        0    -1495     
==========================================
- Hits       111803    98756   -13047     
+ Misses      14039    12006    -2033     
+ Partials      365        0     -365

Impacted Files	Coverage Δ
cpp/src/arrow/dataset/file_base.h	87.5% <ø> (ø)	⬆️
cpp/src/arrow/dataset/dataset.cc	100% <ø> (ø)	⬆️
cpp/src/parquet/arrow/reader.h	65% <ø> (ø)	⬆️
cpp/src/arrow/table.h	100% <ø> (ø)	⬆️
cpp/src/arrow/dataset/scanner.h	100% <ø> (ø)	⬆️
cpp/src/parquet/arrow/reader.cc	83.5% <ø> (+2.7%)	⬆️
cpp/src/arrow/dataset/dataset_internal.h	100% <ø> (ø)	⬆️
cpp/src/arrow/dataset/file_parquet.h	87.5% <ø> (ø)	⬆️
cpp/src/arrow/record_batch.cc	94.07% <100%> (ø)	⬆️
cpp/src/arrow/dataset/file_parquet.cc	100% <100%> (ø)	⬆️
... and 207 more

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[kszucs]

I have not seen the failures, everything seem to pass now though.

[pitrou]

+1