Make __getitem__ block to download shard if not present.

[knighton]

Description of changes:

Make StreamingDataset[sample_id] block to download the given sample's shard if it is not present, so that the dataset can be used lazily.

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[dakinggg]

LGTM, have you done any manual testing to test the scenario where the locks are necessary (i.e. one worker accesses while another is downloading the necessary shard)? That sounds like a fairly complex unit test to write, but would be good to somehow verify that functionality. Your comments in this PR is also making me think that we should have some basic regression testing for streaming...