Generate Unique Suffix (version 2)

[sandrask]

Created to address following comment:

Even though protocol supports the list of multihashing algorithms in this protocol version (v1) we can have only one multihashing algorithm. Later versions may have multiple values for backward compatibility. At that point (version 2) the spec will hopefully better define how to handle this scenarios.

https://github.com/decentralized-identity/sidetree/issues/965