Add Reader class and refactor shield sync

[Duddino]

Abstract

Refactor shield sync by introducing a Reader class.
This allows us to split the reader logic with the actual sync logic, and makes unit testing the former easier.
It makes syncing shield possible when content-length is wrong or empty, useful in e2e testing since the content-length header is removed when doing playback.
It also makes shield sync more consistent across different browsers in terms of when to split chunks, as previously we just relied on the browser (for example on my machine firefox was using much larger chunks than chrome, making the progress bar jump more)

Testing

• Sync shield and assert that it works

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[panleone]

tACK cdfbdac