Add PRICE_UPDATE_THRESHOLD to config, add sanity check

[kilrau]

#77 implemented a 0.1% PRICE_UPDATE_THRESHOLD, meaning arby won't update orders on OpenDEX side if price on CEX side moves less than 0.1%. Previously it would update orders with every single price move, causing quite a flood of order updates on the network.

This follow-up issue is about:

• adding PRICE_UPDATE_THRESHOLD to the [arby] config section
• add a check that prevents arby from starting, logging an error when PRICE_UPDATE_THRESHOLD > margin since that would allow losses for the market maker