Change the usage of `withOriginTH` in TPraos to `withOrigin`

[jasagredo]

We happen to use a different encoding for WithOrigin that we only use in the TPraos header when serializing snapshot. We should change it to the same that we use elsewhere, but we should do it when the ledger publishes an incompatible serialization change.

We should also consider incrementing the version number in the snapshotting format for headers.