- Replace int with type safe BlockHeight(Long) object
- Fix down casting bug, as BlockHeight is uint32 but Java only supports int32 or int64
- Rename WalletBirthday to Checkpoint and hide from the public API
Simplifies running the SDK test suite versus darkside tests (which require special environment setup).
Note that the darkside tests are still broken. This is not a regression, as they were broken before. This is an intermediate step towards fixing those tests.