Convert context cancelation to fetch abort

[flimzy]

Needs tests.

Fixes #666

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.50%. Comparing base (3da3849) to head (55866f2).
⚠️ Report is 1519 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #726   +/-   ##
=======================================
  Coverage   82.50%   82.50%           
=======================================
  Files          85       85           
  Lines        9009     9009           
=======================================
  Hits         7433     7433           
  Misses       1368     1368           
  Partials      208      208           

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