Ensure new sessions will never be found when looking for old ones

This alters the key format for new sessions to ensure that if using the
wildcard form to look for all sessions for a user, new JSON sessions
will never be returned accidentally. This might otherwise have been the
case for users named `v2`

app/models/active_session.rb

@@ -162,7 +162,7 @@ class ActiveSession
   end
 
   def self.key_name(user_id, session_id = '*')
-    "#{Gitlab::Redis::Sessions::USER_SESSIONS_NAMESPACE}:v2:#{user_id}:#{session_id}"
+    "#{Gitlab::Redis::Sessions::USER_SESSIONS_NAMESPACE}::v2:#{user_id}:#{session_id}"
   end
 
   # Deprecated