PTFM-18402 Download chunk size too small
PTFM-18402 Download chunk size too small

Summary:
The download chunk size is currently set to dxfile.MIN_BUFFER_SIZE, but we want
the chunk size to be dxfile.DEFAULT_BUFFER_SIZE

Test Plan: upload/download client tests

Reviewers: orodeh

Reviewed By: orodeh

Differential Revision: https://dnanexus.phacility.com/D1042

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