Make kuzzle.query return the full response object from Kuzzle backend

[ballinette]

What does this PR do?

Make kuzzle.query return value consistent with other sdks, and give access to the whole Kuzzle response.

Given we have following response from Kuzzle:

{
  "status": 200,
  "error": null,
  "result": {"foo": "bar"},
  "volatile": {"you": "'win"}
}

Before this PR:

• kuzzle.query resolved only the result ({foo: 'bar'})

• Some actions resolved directly the result from Kuzzle.query
  example: server.getAllStats (https://github.com/kuzzleio/sdk-javascript/blob/6.x/src/controllers/server.js#L49 ):

return this.kuzzle.query({
  controller: 'server',
  action: 'getAllStats'
}, options);

• Some other parsed the query result to resolve the pertinent value (exemlpe
  example: server.now (https://github.com/kuzzleio/sdk-javascript/blob/6.x/src/controllers/server.js#L118):

return this.kuzzle.query({
  controller: 'server',
  action: 'now'
}, options)
  .then(result => result.now);

Now:

• kuzzle.query act as a low-level method which resolves the whole response from the backend, giving access to the response's status, the volatile data, etc.

• Consequently, each controller's actions must parse the response to resolve the response.result object.
  Examples:

server.getAllstats:

return this.kuzzle.query({
  controller: 'server',
  action: 'getAllStats'
}, options)
  .then(response => response.result);

server.now:

return this.kuzzle.query({
  controller: 'server',
  action: 'now'
}, options)
  .then(response => response.result.now);

[codecov-io]

Codecov Report

Merging #305 into 6.x will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             6.x     #305      +/-   ##
=========================================
+ Coverage   97.5%   97.64%   +0.13%     
=========================================
  Files         28       28              
  Lines       1325     1400      +75     
=========================================
+ Hits        1292     1367      +75     
  Misses        33       33

Impacted Files	Coverage Δ
src/controllers/realtime/room.js	100% <100%> (ø)	⬆️
src/controllers/bulk.js	100% <100%> (ø)	⬆️
src/controllers/index.js	97.36% <100%> (+0.81%)	⬆️
src/controllers/server.js	100% <100%> (ø)	⬆️
src/Kuzzle.js	99.29% <100%> (-0.01%)	⬇️
src/controllers/document.js	100% <100%> (ø)	⬆️
src/controllers/searchResult/base.js	100% <100%> (ø)	⬆️
src/controllers/collection.js	100% <100%> (ø)	⬆️
src/controllers/memoryStorage.js	96.29% <100%> (ø)	⬆️
src/controllers/security/index.js	100% <100%> (ø)	⬆️
... and 2 more

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