PARSEC credential cache

on connecting a client needs to

1. get "change plugin packet" from the server to switch to
2. compute the private key, PBKDF2 is intentionally slow, see this comment
3. request the ext-salt from the server, it costs a round-trip.

The client could keep a local in-memory cache, mapping <username, password, host> key to the <default plugin, private key, ext-salt> value. This way it'll avoid both overhead drastically reducing the connection time. Must work for multi-threaded clients too.

When done, server's parsec_iterations can be increased to 8 by default.