managedUnsubscribe (channel, identifier) {
    const s = this.getSocketWithSubRef(channel, identifier)

    if (!s) {
      debug('cannot unsub from unknown channel %s: %s', channel, identifier)
      return
    }

    const chanId = s.ws._chanIdByIdentifier(channel, identifier)
    s.ws.managedUnsubscribe(channel, identifier)
    s.pendingUnsubscriptions.push(chanId)
  }
@param {string} channel - channel type
@param {string} identifier - unique channel identifier