Standardize memory storage object

[scottinet]

Description

Make the MemoryStorage object on par with the newly refactored corresponding controller.

[codecov-io]

Codecov Report

Merging #191 into 4.x will decrease coverage by -0.04%.
The diff coverage is 98.95%.

@@            Coverage Diff             @@
##              4.x     #191      +/-   ##
==========================================
- Coverage   99.45%   99.41%   -0.04%     
==========================================
  Files          16       16              
  Lines        1648     1709      +61     
  Branches      440      459      +19     
==========================================
+ Hits         1639     1699      +60     
- Misses          9       10       +1

Impacted Files	Coverage Δ
src/MemoryStorage.js	99.09% <98.95%> (+1.13%)	✅
src/Kuzzle.js	99.45% <0%> (-0.19%)	❌

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