cache password hash in SASL auth

[matej21]

In SASL auth handling, Hi function is called for every established connection although the password and hash are same. In my measurements, it takes 10-20ms to calculate the hash with default 4096 iterations. Caching the result of this function would significantly improve performance.
What do you think? I can prepare PR.

[matej21]

Just find out, Hi function can be replaced with crypto.pbkdf2Sync(password, salt, 4096, 32, 'sha256'), which is like 20 times faster..