Recreating a resource while it is being deleted

[Tasko Olevski]

This was uncovered by the integration tests at renku-notebooks.

Steps to reproduce:

• create a server
• delete it with propagation policy set to foreground
• try to recreate the same resource
• things just get stuck - you get no error but you also do not get eventually a new session

We should get an error or the session should be restarted.