Rework token definitions and locked balances

[aaroncox]

The currency stats calls should look for locked balances on a small number of accounts, probably defined in the config table. Any balances on these accounts (eosio, eosio.null) are subtracted from the circulating supply and are considered locked/frozen/burned.

The native API calls on Unicove just moved to this:

greymass/unicove@670d5ec

Also probably a good opportunity to mirror the response types between the token revamp on Unicove and how the contract responds with information about tokens.