UMIP 7 - Add BTC/USD and USD/BTC as price identifiers (DEPRECIATED)

The USDBTC price identifier has been deprecated. It is highly recommended that contract deployers do not use this identifier in its current state for new contracts. Price requests for this identifier from contracts created after 02/26/21 00:00 UTC will likely not be resolved correctly. Price requests from contracts created before this time will not be impacted.

The DVM should support price requests for the BTC/USD and USD/BTC price indexes.