Unify account and nickname; implement PASS; always require auth

- This is now no longer a traditional IRCd, in that registering a connection
  requires you to log into an account (and you're forcibly assigned the nickname
  of that account upon doing so).
  - (Accounts are stored in the database.)
- You can authenticate to this account via either SASL or using the PASS
  command.
- A bug where errors in the client thread would cause an infinite loop was
  fixed.