Use iterator in access token migration

[acidtv]

Fixes#1521

Description of the Change

This PR uses .iterator() instead of .all() in the 0012_add_token_checksum migration to prevent reading the whole table into memory.

I'm not sure if it makes sense to unittest this.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.38%. Comparing base (e34819a) to head (735c3fe).
Report is 29 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #1522 +/- ## ========================================== - Coverage 97.41% 97.38% -0.04%  ========================================== Files 34 34 Lines 2164 2214 +50 ========================================== + Hits 2108 2156 +48 - Misses 56 58 +2 

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[n2ygk]

Thanks!