Refactor some download code for readability + ensure that we checksum resumed downloads

[yarikoptic]

• Closes #1528

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 38.77551% with 30 lines in your changes missing coverage. Please review.

Project coverage is 88.52%. Comparing base (ff49fd1) to head (7efa0b2). Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
dandi/download.py	23.07%	30 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1535 +/- ## ========================================== + Coverage 88.44% 88.52% +0.07% ========================================== Files 78 78 Lines 10691 10713 +22 ========================================== + Hits 9456 9484 +28 + Misses 1235 1229 -6 ``` | [Flag](https://app.codecov.io/gh/dandi/dandi-cli/pull/1535/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/dandi/dandi-cli/pull/1535/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | `88.52% <38.77%> (+0.07%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#carryforward-flags-in-the-pull-request-comment) to find out more.

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[github-actions[bot]]

:rocket: PR was released in 0.65.0 :rocket: