Fix bug when seeking in a non-seekable source stream leading to boundary crossings.

[pontus]

This fixes a bug where the position in the underlying data stream was not properly updated when seeking forward in a non-seekable stream, which could lead to incorrect data returned. A related possible issue with delayed update of stream position after read is also fixed.

This PR also makes almost all test run for both seekable and non-seekable data source streams.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8b63ead) 66.46% compared to head (a6128d8) 65.85%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   66.46%   65.85%   -0.62%     
==========================================
  Files           6        6              
  Lines        1184     1186       +2     
==========================================
- Hits          787      781       -6     
- Misses        278      284       +6     
- Partials      119      121       +2     

Flag	Coverage Δ
unittests	65.85% <100.00%> (-0.62%)	⬇️

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[dbampalikis]

Good job! Tested with the sda-download and fixes that issue too!

[aaperis]

Nice!