_flushOrderOps () {
    this._orderOpTimeout = null

    const packets = this._orderOpBuffer.map(p => [p[1], p[3]])
    this._orderOpBuffer = []

    if (packets.length <= 15) {
      return this.submitOrderMultiOp(packets)
    }

    const promises = []

    while (packets.length > 0) {
      const opPackets = packets.splice(0, Math.min(packets.length, 15))
      promises.push(this.submitOrderMultiOp(opPackets))
    }

    return Promise.all(promises)
  }
Splits the op buffer into packets of max 15 ops each, and sends them down
the wire.

@returns {Promise} p - resolves after send
@private