SDK Refactor: server controller

[ballinette]

• Added Server controller with the following methods:
  • adminExists()
  • getAllStats()
  • getConfig()
  • getLastStats()
  • getStats()
  • info()
  • now()

fix https://github.com/kuzzleio/kuzzle-backlog/issues/101

[codecov-io]

Codecov Report

Merging #285 into 6.x will increase coverage by 2.27%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              6.x    #285      +/-   ##
=========================================
+ Coverage   84.13%   86.4%   +2.27%     
=========================================
  Files          20      21       +1     
  Lines        1973    1957      -16     
=========================================
+ Hits         1660    1691      +31     
+ Misses        313     266      -47

Impacted Files	Coverage Δ
src/Kuzzle.js	54.61% <100%> (+8.75%)	⬆️
src/Server.js	100% <100%> (ø)

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[benoitvidis]

I don't think we need bluebird to promisify functions.
It would be simpler imo to directly modify query and its dependencies to return a promise (wip on my side)

[ballinette]

@benoitvidis As explained in the PR's description, I let the bluebird's promisification for query here only to keep the whole code working until the refactoring is not totally finished yet:

If we modify first the query method, all methods that use it will be broken. So we have 2 options:

• either have a broken code until we have rewriten all the SDK (and be sure we won't draft any new release before that)
• or keep query as is for now and rewrite it at the final step of the refactoring.

I personally chose the 2nd option because IMHO, it's a better way for me to make sure each PR don't break methods that are still not rewritten...

[ballinette]

@benoitvidis
Ok, since #288 has been merged and already contains the refactor of query method that you made, I moved to it in this one, and resovled merge conflicts.

@jenow you may want to review this PR again.

[ballinette]

replaced by #290