User db storage + login/pwd db using system tables.
Authenticator object is a global shard-shared singleton, assumed
to be completely immutable, thus safe.
Actual login authentication is done via locally created stateful object
(sasl challenge), that queries db.
Uses "crypt_r" for password hashing, vs. origins use of bcrypt.
Main reason is that bcrypt does not exist as any consistent package
that can be consumed, so to guarantee full compatibility we'd have
to include the source. Not hard, but at least initially more work than
worth.
