Support ms:mexecute

[Leodau]

What does this PR do?

Implements support for the newly added ms:mexecute

How should this be manually tested?

const actions = [
    { 'action': 'set', 'args': { '_id': 'list:a', 'body': { 'value': 1, 'ex': 100, 'nx': true } } },
    { 'action': 'get', 'args': { '_id': 'list:a' } },
    { 'action': 'del', 'args': { 'body': { 'keys': ['list:a'] } } }];

  // Should Print : "[ [ null, 'OK' ], [ null, '1' ], [ null, 1 ] ]"
  console.log(await kuzzle.ms.mexecute(actions));

Boyscout

• Applied one-var esrule in the memoryStorage file.

[codecov]

Codecov Report

Merging #524 into 7-dev will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            7-dev     #524   +/-   ##
=======================================
  Coverage   95.59%   95.59%           
=======================================
  Files          32       32           
  Lines        1318     1318           
=======================================
  Hits         1260     1260           
  Misses         58       58           

Impacted Files	Coverage Δ
src/controllers/MemoryStorage.js	96.25% <100.00%> (ø)

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