Bug: Session should be updated in values sorted set on save

Currently the active session stat in the colonel is not accurate. It counts new sessions in the past 15 mins but even if a session stays active, it will "fall out" of the most recent 15 minutes b/c its score is never updated.

Updating the score in the values sorted set on Session#save may be too aggressive. Doing it in validate-shrimp perhaps? Or the auth-check might be more efficient.