unsubscribe (chanId) {
    const s = this.getSocketWithChannel(chanId)

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

    s.ws.unsubscribe(chanId)
    s.pendingUnsubscriptions.push(chanId)
  }
Unsubscribes the first socket w/ the specified channel. Does nothing if no
such socket is found.

@param {number} chanId - channel ID