New "currency" field on journal request schemas

Broker API

Journal requests now support a per-journal currency.

Adds an optional currency field to the reverse batch journal request schema.
currency defaults to the system currency (for example, USD in the US or EUR in the EU) when omitted. Correspondents with local currency trading enabled must send this field, and the value must be the system currency or one of the correspondent's enabled currencies. All entries in a batch should use the same currency.
amount is now documented as the journal amount in the entry's currency rather than USD.