Update QUOTE_ASSETS to reflect current DEX realities

[droplister]

The counterblockd code defines:
QUOTE_ASSETS = ['BTC', 'XBTC', 'XCP'] # define the priority for quote asset

This configuration value is used by the assets_to_asset_pair utility function to deal with order asset pairs in a deterministic way and is a fair solution to the problem of, "Which asset comes first, 1 or 2?" But when an asset is not listed as part of the priority quote assets, the function defaults to alphabetical order which can create some unexpected results and inverted prices.

Today, I believe it makes sense to update this value to give PEPECASH and BITCRYSTALS priority as quote assets in the default configuration. This will improve the user experience of Counterwallet, as well. Additional candidates could include, RUSTBITS, SCOTCOIN, DATABITS, and hopefully one day BITCORN.

Perhaps, we could create a simple threshold in terms of trading volume to include assets here. Node operators could, of course, update this configuration value without causing any consensus issues, so I would encourage that this is done, but most node operators do not edit these default values.

XBTC hasn't been traded on the dex since 2014. I think it's worth updating this default config value to include PEPECASH and BITCRYSTALS, based on their clear volume of usage, and to talk about what threshold could be put in place to regularly be adding quote assets for priority.

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[droplister]

I talk about this issue in this video @ 10m30s:
https://www.youtube.com/watch?v=Sai0Eh7lpec#t=10m30