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ARROW-11596: [Python][Dataset] make ScanTask.execute() eager

This changes 2 things:
- ScanTask.execute() is now eagerly evaluated, so any work involved in creating a record batch reader is done up front. For example, a Parquet file will actually begin reading (but not decoding) data. This mirrors the behavior in C++. This also makes it easier to separate stages of work when pipelining reads (by manually dispatching scan tasks).
- This has the side effect of working around a SIGSEGV in the Cython generator, caused because Cython raises StopIteration without holding the GIL when you have a generator that uses "with [no]gil". In my tests this is fixed on Cython master but not any stable or development release.

Closes #9480 from lidavidm/arrow-11596

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>