'Returns a hash value of the list of block records.
This currently hashes using sha1, and returns a base64 encoded version
of the binary digest.  In the future, different algorithms could be
supported by adding a label indicated which algorithm was used, e.g.,
"sha256$j0NDRmSPa5bfid2pAcUXaxCm2Dlh3TwayItZstwyeqQ=".'

@lazy
def hash_value(self):

    return b64encode(sha1(self.json_value).digest())
