Since the enum is part of the BlockHeader interface, it does not need
    a long name to relate to block.

    Also renamed the enum items as follows:
    - SEAL_NA to NOT_APPLICABLE
    - SEAL_POW_BLOCK to PROOF_OF_WORK
    - SEAL_POS_BLOCK to PROOF_OF_STAKE
    This remaning is meant to reduce visual confusion when dealing with
    the POS and POW seal types.