Refactor bank keeper by removing private funcs

[fedekunze]

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4663 into master will decrease coverage by 0.02%.
The diff coverage is 74.66%.

@@            Coverage Diff             @@
##           master    #4663      +/-   ##
==========================================
- Coverage   55.06%   55.03%   -0.03%     
==========================================
  Files         273      273              
  Lines       17086    17066      -20     
==========================================
- Hits         9408     9392      -16     
+ Misses       6984     6981       -3     
+ Partials      694      693       -1

[alexanderbez]

ACK ☕️