Allow passing a custom network object to Kuzzle SDK constructor

[ballinette]

What does this PR do?

See KZL-175

Change the signature of SDK constructor to match actual signature in Go SDK and enable developers to instantiate the SDK with their custom network protocol:

// Before: 
const kuzzle = new Kuzzle('hostname', {protocol: 'websocket'});

// Now - with embed protocol:
const kuzzle = new Kuzzle('websocket', {host: 'hostname'});

// Now - with custom protocol:
const protocol = new MyCustomProtocol();
const kuzzle = new Kuzzle(protocol, {});

Other changes

Expose also AbstractNetwork and KuzzleEventEmitter:
Thus, custom protocol can easily inherits from AbstractNetwork and only implements connect and send methods:

const
  Kuzzle = require('kuzzle-sdk').Kuzzle,
  KuzzleAbstractNetwork = require('kuzzle-sdk').KuzzleAbstractNetwork;

class MyCustomProtocol extends KuzzleAbstractNetwork {
  connect() {
    // (...) do custom connection steps...

    // change state and resolve:
    this.state = 'ready';
    return Promise.resolve();
  }

  send(request) {
    // (...) here the protocol-specific code to send the request to kuzzle and get the result into `result` variable

    // Send back the result to SDK and resolve:
    this.emit(request.requestId, {result});
    return Promise.resolve();
  }
}

const protocol = new MyCustomProtocol();
const kuzzle = new Kuzzle(protocol);
kuzzle.connect()
.then(() => kuzzle.server.now())
.then(res => {
  console.log(`Current Kuzzle timestamp: ${res}`);
})

Another way to implement a custom protocol is to implement isReady and query methods, as well as javascript Event API:

const
  Kuzzle = require('kuzzle-sdk').Kuzzle,
  KuzzleEventEmitter = require('kuzzle-sdk').KuzzleEventEmitter;

class MyCustomProtocol extends KuzzleEventEmitter {

  isReady() {
    return true;
  }

  query (request, options) {
    // (...) here the protocol-specific code to send the request to kuzzle and get the result into `result` variable

    // Resolves the response:
    return Promise.resolve({result});
  }
}

Boyscout

Some minor cosmetic changes in network wrappers code.

[codecov-io]

Codecov Report

Merging #294 into 6.x will decrease coverage by 0.25%.
The diff coverage is 75.55%.

@@            Coverage Diff             @@
##              6.x     #294      +/-   ##
==========================================
- Coverage   94.92%   94.66%   -0.26%     
==========================================
  Files          28       28              
  Lines        1319     1331      +12     
==========================================
+ Hits         1252     1260       +8     
- Misses         67       71       +4

Impacted Files	Coverage Δ
src/Kuzzle.js	99.3% <100%> (ø)	⬆️
src/networkWrapper/index.js	100% <100%> (+10%)	⬆️
src/networkWrapper/protocols/http.js	86.2% <33.33%> (-1.89%)	⬇️
src/networkWrapper/protocols/abstract/common.js	58.25% <57.14%> (ø)	⬆️
src/networkWrapper/protocols/socketio.js	94.64% <75%> (-1.66%)	⬇️
src/networkWrapper/protocols/websocket.js	98.07% <75%> (-1.93%)	⬇️
src/networkWrapper/protocols/abstract/realtime.js	94.44% <77.77%> (-2.53%)	⬇️

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