This patch implements an Authenticator type, which connections use to
do authentication and user existence checks.
It simplifies the abstractions (the server doesn't need to know about
userdb, or keep track of domain-userdb maps), and lays the foundation
for other types of authentication backends which will come in later
patches.
This patch implements the AUTH SMTP command, using per-domain user databases.
Note that we don't really use or check the validation for anything, this is
just implementing the command itself.
