fix DiffKVStores(), store/types gets 100% coverage

[alessio]

DiffKVStores() used to return duplicated entries
in some cases.

Add test cases, aiming to reach 100% coverage for
store package.

Remove duplicate Cp function from the store package.
Same functionality is provided by types.CopyBytes().

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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)

[tac0turtle]

if you want to do these on tendermint as well, we'd love it hehe 😄

[codecov]

Codecov Report

Merging #5730 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5730      +/-   ##
==========================================
+ Coverage   36.19%   36.22%   +0.03%     
==========================================
  Files         335      335              
  Lines       32722    32722              
==========================================
+ Hits        11843    11855      +12     
+ Misses      19653    19641      -12     
  Partials     1226     1226

Impacted Files	Coverage Δ
client/keys/types.go	100% <0%> (+100%)	⬆️