stats: Added ADD-USER command to add stats to a user without having a session.

stats: Added ADD-USER command to add stats to a user without having a session.

This will be used by at least the auth process. Although the auth process
does have a session, it's a bit too early to start using it yet. At least the
PID should be possible to change when the session moves to imap process.
Also the unsuccessful authentication sessions shouldn't really be added at
all.