refactor: x/oracle: make GetExchangeRateBase more efficient

[adamewozniak]

Description

Resolves a small efficiency issue with the GetExchangeRateBase function + will add some tests for it

closes: #865

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[codecov-commenter]

Codecov Report

Merging #866 (147035c) into main (87cb558) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #866      +/-   ##
==========================================
+ Coverage   50.93%   51.08%   +0.15%     
==========================================
  Files          64       64              
  Lines        9428     9425       -3     
==========================================
+ Hits         4802     4815      +13     
+ Misses       4383     4365      -18     
- Partials      243      245       +2     

Impacted Files	Coverage Δ
x/oracle/keeper/keeper.go	84.79% <100.00%> (+7.06%)	⬆️

[adamewozniak]

We should follow up on this in #661 by cleaning up this test file a bit - could be table driven