Returns a random hash seed and the block number since which it was determinable by chain observers.
- The subject buffer can be used to further randomize the hash.
- Within the same execution returns the same random hash for the same subject.
If the returned value cannot be properly decoded.
The returned seed should only be used to distinguish commitments made before the returned block number. If the block number is too early (i.e. commitments were made afterwards), then ensure no further commitments may be made and repeatedly call this on later blocks until the block number returned is later than the latest commitment.