Ideally, we want the /.../account/addresses to be paginated,
otherwise there's no real value in isolating it as such. Also,
for now, the implementation is rather heavy and requires the
full account to be retrieved from the DB. On the long-term, we
could imagine fetching only the addresses from the DB and
streaming the response.

This is not 'ideal' yet because we are sorting and filtering on a plain list.
Instead, we could have these operation at a DB-level, but this won't probably
be achieved with acid-state. Perhaps if one day we switch to SQL

There was no integration tests for any accounts endpoints :s ... We should work on that in the future as I didn't take care of it
in this commit / task, but solely add a few one for the newly added endpoints

Also, this rename the balance endpoint to '/api/v1/wallets/{id}/accounts/{ix}/amount' to reflect the actual name of
the field in the Account representation.