Implement AuthenticationHandler for custom auth mechanisms

[ramnes]

Hey team,

Here is some work around #1069 and #1071 to allow a discussion based on an actual implementation.

I think the changes here are pretty obvious:

• Renames;
• A Credential can have multiple passwords now;
• Added OnAuthSuccess and OnAuthFailure to the new AuthenticationHandler.

One side effect of the multiple passwords per user is that we can't have Xor update inplace anymore, because otherwise we would XOR the password we received every time we compare it to a new hash in the list of hashes to try.

About hashes – I moved the Credential passwords hash at comparison-time to avoid paying the full price for every connection. Let's say you have 1000 passwords to match against but match after 5 comparisons: it's better to hash only 5 passwords, not 1000.

[lance6716]

(This PR is a bit large than the other one you opened, I'll take a look later)

[ramnes]

Yeah, the PR is a bit too large for my taste too. :)

I didn’t mean for it to be merged as-is, more as a starting point for discussion around the issues I opened. Happy to split it up once we have a clearer idea of what we want to do for each one!