This commit makes Renames `WalletBirthday` to `Checkpoint` and
makes Checkpoint an internal type. Public ocurrences of this
type is replaced by `BlockHeight` (Int) and then retrieval of the
Checkpoint is deferred to the appropiate place in the code
Add an extension method to `BlockHeight` to get latest checkpoint
height present in the SDK's bundle
PR Suggestions:
Add test for integer overflow on JSON
File renamed from WalletBirthday to Checkpoint
2fcfa6fdbe