This PS revamps document hashing. Instead of relying on Python's
built-in hash function to hash the contents of a document (i.e.
metadata and data values), sha256 from hashlib is used instead,
mostly for security purposes.
Further, new parameters have been added to the document DB model:
data_hash and metadata_hash, and the old value hash has been
dropped. The data type for storing the hashes has been changed
to String from BigInt.
Finally, testing documentation was added.
Change-Id: I428ddcbce1007ea990ca0df1aa630072a050c722
8bf4f7407d