R4R: Support query txs' TotalCount in GET /txs #3942

[yangyanqing]

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

close: #3942

• 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: sdkch 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 #3995 into develop will decrease coverage by 0.04%.
The diff coverage is 38.46%.

@@             Coverage Diff             @@
##           develop    #3995      +/-   ##
===========================================
- Coverage    60.19%   60.15%   -0.05%     
===========================================
  Files          212      212              
  Lines        15157    15166       +9     
===========================================
- Hits          9124     9123       -1     
- Misses        5409     5419      +10     
  Partials       624      624

[alexanderbez]

Thanks @yangyanqing! Left some feedback, but overall looks solid.