[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)
```
> /^[0-9]*$/.test('') ? Number('') : 'validated'
0
```

Might want to make it [0-9]+ or Just test !isNaN