joinChannel(channel, connectionId) {
    debug('[server] client "%s" joining channel "%s"', connectionId, channel);

    const client = this._clients.get(connectionId);

    if (!client || !client.protocol) {
      return;
    }

    try {
      this.protocols.get(client.protocol).joinChannel(channel, connectionId);
    } catch (e) {
      global.kuzzle.log.error(
        `[join] protocol ${client && client.protocol} failed: ${e.message}`,
      );
    }
  }
On client subscribing to a channel, dispatch the information
to the protocol.

@param {string} channel
@param {string} connectionId