Users: Track user is called multiple times when there is a transfer but the user no longer exists

[JurgenR]

For pending transfers we attempt to track the user using the AddUser command, only if a successful AddUser message is returned is a flag set that indicates we no longer need to send a second message. However if there is a transfer and the user does not exist we will attempt to send an AddUser when doing an transfer management cycle.

If there's many transfer this leads to a lot of AddUser messages for this user. In order to reduce this we should put a delay, for example to retry every 10 minutes