managedUnsubscribe (channel = '', identifier = '') {
    const key = `${channel}:${identifier}`
    const chanId = this._chanIdByIdentifier(channel, identifier)

    if (chanId === null || isNaN(this._subscriptionRefs[key])) return false

    this._subscriptionRefs[key]--
    if (this._subscriptionRefs[key] > 0) return false

    this.unsubscribe(chanId)
    delete this._subscriptionRefs[key]

    return true
  }
Decreases the subscription ref count for the channel/identifier pair, and
unsubscribes from the channel if it reaches 0.

@param {string} channel - channel name
@param {string} identifier - for uniquely identifying the ref count
@returns {boolean} unsubSent