Prevent DOS attacks by pre-hashing password

[dhui]

Hash used for pre-hashing should be configurable. Default to SHA-3.
e.g. Use hash.Hash and sha3.New512() (each hash sum call will either need to lock and reset the hash.Hash OR create a new hash.Hash)

Reference: https://github.com/OWASP/CheatSheetSeries/blob/master/cheatsheets/Password_Storage_Cheat_Sheet.md#hash-the-password-as-one-of-several-steps

[dhui]

Turns out pre-hashing a password can be dangerous. e.g. bcrypt of an MD5 hash can be shucked

It's not clear if SHA3 currently has similar vulnerabilities (e.g. any breaches containing SHA3 hashes) but the potential vulnerability is always there in the future. As the linked video mentions, this can be mitigated by peppering the pre-hashed password.