ARROW-10046: [Rust] [DataFusion] Made `RecordBatchReader` implement Iterator#8225

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jorgecarleitao:reader_iterator

Commits on Sep 30, 2020

Migrated next_batch from Result<Option<>> to Option<Result<>>
jorgecarleitao
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Added RecordBatch Iterator to all execution nodes.
jorgecarleitao
committed

Commits on Oct 1, 2020

Added next_batch with a deprecation warning.
jorgecarleitao
committed