Add a converter for CurrencyUnits stored in the database (#334)

* Add a converter for CurrencyUnits stored in the database

This uses the well-known String representation for currency units. It also
provides a base class for other converters that will be persisting the
toString() representation.

* Add DB and formatting changes

* Add tests, make minor fixes

db/src/main/resources/sql/schema/db-schema.sql.generated

@@ -133,7 +133,7 @@
        revision_id  bigserial not null,
         bloom_filter bytea not null,
         creation_timestamp timestamptz not null,
-        currency bytea not null,
+        currency text not null,
         name text not null,
         primary key (revision_id)
     );