mm: Handle market params update

[martonp]

This diff updates the market maker to listen for updates to the server config. If the lot size of a market that a bot is currently trading on is updated, the number of placements that the bot makes is updated so that the bot places as many lots as possible without exceeding the amount that was placed before the lot size update.

Additionally, the lot size is now saved with the BotConfig. An additional overrideLotSizeChange parameter is added to StartBot, which if false, will return an error if the lot size has changed since the bot was configured. This will allow the UI to prompt the user to either update their configuration, or continue with the old one even though the lot size has changed.

[buck54321]

Let's say we have 4 sell placements of 1 lot each. If we double the lot size, this will result in 4 placements of 0 lots. Instead, I think we should focus on preserving total quantity and prioritizing higher-priority placements, so that we end up with 2 placements of 1 lot instead.

func (c *ArbMarketMakerConfig) updateLotSize(originalLotSize, newLotSize uint64) error {
	updatedPlacements := func(oldPlacements []*ArbMarketMakingPlacement) (newPlacements []*ArbMarketMakingPlacement) {
		var qtyCounter uint64
		for _, p := range oldPlacements {
			qtyCounter += p.Lots * originalLotSize
		}
		newPlacements = make([]*ArbMarketMakingPlacement, 0, len(oldPlacements))
		for _, p := range oldPlacements {
			lots := utils.Max((p.Lots*originalLotSize)/newLotSize, 1) // might want to round instead of truncate
			qty := lots * newLotSize
			if qty > qtyCounter {
				return
			}
			qtyCounter -= qty
			newPlacements = append(newPlacements, &ArbMarketMakingPlacement{
				Lots:      lots,
				Multiplier: p.Multiplier,
			})
		}
		return
	}

	if len(c.SellPlacements) > 0 {
		c.SellPlacements = updatedPlacements(c.SellPlacements)
		if len(c.SellPlacements) == 0 {
			return errors.New("no sell placements possible with new lot size")
		}
	}
	if len(c.BuyPlacements) > 0 {
		c.BuyPlacements = updatedPlacements(c.BuyPlacements)
		if len(c.BuyPlacements) == 0 {
			return errors.New("no buy placements possible with new lot size")
		}
	}
	return nil
}

Either way, we need to validate the resulting placements to ensure there are none with zero lots, especially since ArbMarketMakerConfig doesn't have a Validate method.