Remove auto connect

[scottinet]

⚠️ depends on #265 ⚠️

Description

Remove the ability to connect automatically from Kuzzle's main object constructor:

• a constructor should always avoid side effects, especially I/O
• this removes the confusion about the callback passed to Kuzzle constructor but called only by kuzzle.connect(): the constructor does not take a callback anymore, but connect does
• kuzzle.connect can now be promisified into kuzzle.connectPromise, as many users expected from reading our documentation
• kuzzle.connect now returns nothing, as it makes no sense chaining calls on an asynchronous network connection method
• this makes the connection workflow much more clearer

Note: a documentation update will be submitted shortly

[codecov-io]

Codecov Report

Merging #266 into 6.x will increase coverage by 0.18%.
The diff coverage is 97.26%.

@@            Coverage Diff             @@
##              6.x     #266      +/-   ##
==========================================
+ Coverage   98.27%   98.46%   +0.18%     
==========================================
  Files          17       17              
  Lines        2140     2079      -61     
  Branches      623      604      -19     
==========================================
- Hits         2103     2047      -56     
+ Misses         37       32       -5

Impacted Files	Coverage Δ
src/networkWrapper/protocols/websocket.js	100% <100%> (ø)	⬆️
src/networkWrapper/protocols/abstract/realtime.js	98.63% <100%> (ø)	⬆️
src/networkWrapper/protocols/socketio.js	94.82% <91.66%> (-5.18%)	⬇️
src/eventEmitter/index.js	97.14% <96.22%> (+4.66%)	⬆️
src/Kuzzle.js	97.84% <97.76%> (+0.31%)	⬆️

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