Save and load exchange rates with persistent JSON file

[JBetz]

Resolves #12.

The exchange rates map is now persisted in <datadir>/exchangerates.json whenever it is updated via the new setfeeexchangerates RPC.

Also, the server parameter exchangeratesjsonfile has been renamed to initialexchangeratesjsonfile to distinguish it from the database file. If the initialization file isn't provided and the database file doesn't exist, it will be created at startup with a default valuation for the policy asset. And if the initialization file is provided and the database file already exists, it will be overwritten with the values in the initialization file. In any case, the initialization file will never be modified by the node unless it resolves to the same path as the database file.

I also switched to using as much of the existing functionality for saving and loading settings found in <util/settings.h>, since settings are also persisted as a JSON file in <datadir>. The banlist database also uses the same mechanisms.

[JBetz]

Added some functional tests which verify that 1) the initial exchange rates are loaded from the initial exchange rates JSON file when provided, 2) the in-memory and on-disk exchange rates are kept in synch whenever the exchange rates are updated via the setfeeexchangerates RPC, 3) the RPC returns a helpful error message when the inputs fail to parse, and 4) that the exchange rates aren't modified when this happens.