Share Redis connection for non-hanging Nucleus Datastore

[sebastienfilion]

It would be interesting to attempt to reduce the number of Redis connection in a given controlled ecosystem. If one or many engines are instantiated on the same node process, there is no point of having multiple Redis connection since they are both single threaded.
Hanging connection like those sued to subscribe to Redis commands or events should not share that connection for obvious reason.

class NucleusDatastore {
  constructor (datastoreName = 'Untitled', options = {}) {
    const {
      $logger = console,
      $$server,
      index: datastoreIndex = 0,
      port: datastorePort = 6379,
      URL: datastoreURL = 'localhost'
    } = options;

    this.name = datastoreName;
    this.index = datastoreIndex;

    this.$$handlerCallbackListByChannelName = {};
    this.scriptSHAbyScriptName = {};

    this.$$server = $$server || redis.createClient({
      db: datastoreIndex,
      host: datastoreURL,
      port: datastorePort
    });
  // ...
  }
}