async submitOrderMultiOp (opPayloads) {
    if (!this._isAuthenticated) {
      throw new Error('not authenticated')
    }

    // TODO: multi-op tracking
    this.send([0, 'ox_multi', null, opPayloads])
  }
Sends the op payloads to the server as an 'ox_multi' command. A promise is
returned and resolves immediately if authenticated, as no confirmation is
available for this message type.

@param {object[]} opPayloads - order operations
@returns {Promise} p - rejects if not authenticated