Problem: concurrent write base fee in fee history (backport: #364)

[mmsqe]

for more info

Closes: #XXX

Description

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For contributor use:

• 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.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #366 (bf383bd) into release/v0.20.x-cronos (3fdb18b) will decrease coverage by 0.01%.
The diff coverage is 37.50%.

Additional details and impacted files

@@                    Coverage Diff                     @@
##           release/v0.20.x-cronos     #366      +/-   ##
==========================================================
- Coverage                   52.54%   52.53%   -0.01%     
==========================================================
  Files                         114      114              
  Lines                       11124    11129       +5     
==========================================================
+ Hits                         5845     5847       +2     
- Misses                       5029     5032       +3     
  Partials                      250      250              

Files	Coverage Δ
rpc/backend/backend.go	50.00% <100.00%> (+3.33%)	⬆️
rpc/backend/chain_info.go	8.45% <0.00%> (-0.13%)	⬇️