Fixed node controller assignemnts update after pods restart

[merlimat]

If a storage node restarts and its IP stop responding, the coordinator health check will detect it in a ~2seconds period, though the component that sends the cluster assignements updates will not detect the issue.

One of the reason is that it's using a grpc stream, so there is no timeout on the send operation (because it would apply to the lifetime of the stream itself, which instead needs to be long-lived).

The storage servers need to receive the new cluster assignments after restart, otherwise they cannot pass it back to their clients.

To fix the issue, we close the assignments stream whenever the health-check failure is triggered. This will ensure that coordinator will retry after the restart.