fix: exception on concurrent download of `ParseFile` from multiple threads

[rommansabbir]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

Related issue: https://github.com/parse-community/Parse-SDK-Android/issues/1155

Approach

This PR simplifies the logic of fetchAsync method and then adds a concurrent code that blocks the second (and further) calling threads until the first thread finishes the download of ParseFile (or fails to do so).

TODOs before merging

• [x] Add tests
• [x] Add changes to documentation (guides, repository pages, in-code descriptions)
• [x] A changelog entry is created automatically using the pull request title (do not manually add a changelog entry)

[parse-github-assistant[bot]]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov[bot]]

Codecov Report

Merging #1180 (ec87160) into master (351858c) will not change coverage. The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master   #1180   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files         122     122           
  Lines        9962    9971    +9     
  Branches     1343    1345    +2     
======================================
- Misses       9962    9971    +9     

Impacted Files	Coverage Δ
...e/src/main/java/com/parse/ParseFileController.java	0.00% <0.00%> (ø)

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[rommansabbir]

@mtrezza @L3K0V How to solve 'codecov/project' failing issue?

[azlekov]

@mtrezza can you help about failing checks

[parseplatformorg]

🎉 This change has been released in version 4.1.0