Delta XDS Fails to Apply Correct Configuration Upon Reconnecting to Control Plane

I am working on implementing delta XDS using a java control plane. I am using the `setSnapshot` method in SimpleCache to send the dynamic configuration to the envoy proxy over GRPC. The `api_type` in the envoy configuration is set to `DELTA_GRPC`.

## Steps to Reproduce the Issue
1. Start the xDS server.
2. Create an Envoy proxy with a bootstrap configuration to connect to the xDS server.
3. Submit a snapshot using `SimpleCache.setSnapshot` with the cluster name `cluster-x`.
4. Verify that the configuration dump details for `cluster-x` are present as expected.
5. Shut down the xDS server.
6. Restart the xDS server.
7. Submit another snapshot using `SimpleCache.setSnapshot` with the cluster name `cluster-y`.
8. Verify that the configuration dump details for both `cluster-x` and `cluster-y` are present.

## Expectations
The cluster named `cluster-x` should have been removed. This behavior is observed when the xDS server state remains unchanged without restarting.