Create user access token from hash

[cxxman]

@cxxman for this issue (#144), I think we must be able to create tokens from hashed values. e.g.

alter user UUU add token TTT from hash x'<hashed_value>';

Then we can export users and import including the initial token. Make sense?

Originally posted by @niolap in https://github.com/siodb/siodb/issues/144#issuecomment-828195982