catchpoints: move logic in readDatabaseStep to reusable catchpointOnlineAccountsIterWrapper

[cce]

Summary

Fix label generation issue detected in nightly e2e tests after merging #6240. Makes the logic reusable for both catchpoint label trackers (CatchpointTracking: 1) and catchpoint file generators (CatchpointTracking: 2) as well as tests.

Test Plan

Existing tests pass now — additional tests to build on #6224 and further assert contents of catchpoint files will follow in a separate PR.

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 6 lines in your changes missing coverage. Please review.

Project coverage is 51.78%. Comparing base (2f07e60) to head (bc0053d).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ledger/catchpointfilewriter.go	76.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6253   +/-   ##
=======================================
  Coverage   51.77%   51.78%           
=======================================
  Files         644      644           
  Lines       86494    86511   +17     
=======================================
+ Hits        44781    44797   +16     
+ Misses      38843    38841    -2     
- Partials     2870     2873    +3     

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[jannotti]

Makes sense to me. Tests passing is probably the best possible code review.