Refactor loading fee asset rates from JSON

[fare]

As a followup to #10 we should refactor the set fee asset rates code paths so that

1. The loading from file and loading from RPC share the same underlying function.
2. That function validates the entire data structure before it is used to atomically modify the global exchangeRateMap, so that the variable is not left in unusable state if something goes wrong.
3. Maybe for persistence, in the RPC variant, after the data is validated and before or after the global variable is updated, save the json to a temporary file, fdatasync, then atomically replace the startup file.