[FEATURE] Balances|Trustlines: Return ledger_index, use last validated ledger by default (RLJS-127)

Also, returns `validated` key as a boolean (from the rippled response)
https://github.com/ripple/ripple-rest/blob/develop/schemas/UINT32.json

Maybe `validator.isValid(request.query.ledger, 'UINT32')`