Improve fetch bookkeeping

[soininen]

fetch_all() was calling do_fetch_more() directly, but it makes more sense to call do_fetch_all() instead to keep record of what has been fetched.

No associated issue

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.10%. Comparing base (7e55a21) to head (a172985).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
- Coverage   79.10%   79.10%   -0.01%     
==========================================
  Files          77       77              
  Lines       10076    10078       +2     
  Branches     2140     2141       +1     
==========================================
+ Hits         7971     7972       +1     
  Misses       1755     1755              
- Partials      350      351       +1     

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