hasSubscriptionRef (channel, identifier) {
    const key = `${channel}:${identifier}`
    return !!Object.keys(this._subscriptionRefs).find(ref => ref === key)
  }
Check if a channel/identifier pair has been subscribed too

@param {string} channel - channel type
@param {string} identifier - unique identifier for the reference
@returns {boolean} hasRef
@see WSv2#managedSubscribe