fix group by transaction

[Officeyutong]

There is a bug in the new implementation of group_by_transaction which may split a tx into two parts in the return value. This PR fixes it by reverting it to the previous implementation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.80%. Comparing base (0685266) to head (bb393fb).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #240   +/-   ##
========================================
  Coverage    71.80%   71.80%           
========================================
  Files           27       27           
  Lines         6942     6942           
========================================
  Hits          4985     4985           
  Misses        1957     1957           

Flag	Coverage Δ
unittests	71.80% <ø> (ø)

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