## Problem
While pbkdf2 is a simple algorithm, we should probably use a well tested
implementation
## Summary of changes
* Use pbkdf2 crate
* Use arrays like the hmac comment says
## Checklist before requesting a review
- [X] I have performed a self-review of my code.
- [X] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.
* [proxy] Add SCRAM auth
* [proxy] Implement some tests for SCRAM
* Refactoring + test fixes
* Hide SCRAM mechanism behind `#[cfg(test)]`
Currently we only use it in tests, so we hide all relevant
module behind `#[cfg(test)]` to prevent "unused item" warnings.
