k8s watcher desync

The k8s watcher may experience very severe desync with the k8s API. A deleted session was still in the database (and returned in the sessions list) for several minutes.

```
The k8s resource with metadata K8sObject(name=flora-abd275c11ceb, namespace=renku, cluster=0RENK1RENK2RENK3RENK4RENK5, gvk=GVK(kind='AmaltheaSession', version='v1alpha1', group='amalthea.dev') ... cannot be found.
```