Always return ledger_index as int, not str, in JSON

[mDuo13]

Summary

This is the "breaking change" follow up to #3533 . The API (v2 or higher) should always return ledger_index fields formatted as the same data type, probably numbers/integers, when serialized to JSON.

Motivation

See #3533 (comment) for some examples of places where ledger_index is returned as an string-integer rather than a proper integer.

[Bronek]

Note, #4820 is planned for API version 3 (not 2)

[intelliot]

Update: We decided to include #4820 for API version 2. It is merged as of 2.0.0-rc4 (and later).