getSocketWithChannel (chanId) {
    return this._sockets.find(s => {
      return (
        s.ws.hasChannel(chanId) &&
        !_includes(s.pendingUnsubscriptions, chanId)
      )
    })
  }
NOTE: Cannot filter against pending subscriptions, due to unknown chanId

@param {number} chanId - channel ID
@returns {object} wsState - undefined if not found