postgres_scram hash support
Created originally on Bitbucket by Anonymous
Postgres 10 will be adding a new scram-sha256 hash. This should be pretty easy to implement given the existing passlib.hash.scram library.
- get some test vectors for postgres' scram hash format.
- clean up existing scram hash internals so we can subclass out common bits.
- modify existing scram hash to support "client" vs "server" hashes, which store just the required subkeys.