refactor(core-state): merge index and reindex into one function inside WalletRepository

[bertiespell]

Summary

Condense index and reindex into one method.

Only one index method means the naming is better. Smaller API (public methods on class) = simpler code.

The index and reindex methods were condensed into one method - index, which accepts either a wallet or an array of wallets.

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

Merging #3563 into develop will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #3563      +/-   ##
===========================================
- Coverage    56.89%   56.89%   -0.01%     
===========================================
  Files          548      548              
  Lines        13661    13663       +2     
  Branches      1867     1868       +1     
===========================================
  Hits          7773     7773              
- Misses        5859     5861       +2     
  Partials        29       29              

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