EDS multiplexing causes EDS initial fetch failure

*Description*:

We're running a GRPC SoTW control plane for xDS. When we deployed a changeset which included https://github.com/envoyproxy/envoy/pull/22419, Envoy failed to fetch EDS from the control plane at all, timing out on initial EDS fetch. When we disabled the feature with the runtime flag `envoy.reloadable_features.multiplex_eds` and restarted Envoy, EDS worked properly again.

Apart from EDS failures, we saw many occurrences of this log message which has never come up before (looks like maybe 1 per EDS cluster):

```
[warning][config] [external/envoy/source/common/config/grpc_stream.h:65] gRPC bidi stream to envoymanager for rpc StreamEndpoints(stream .envoy.service.discovery.v3.DiscoveryRequest) returns (stream .envoy.service.discovery.v3.DiscoveryResponse); already exists!
```

The PR was reverted in https://github.com/envoyproxy/envoy/pull/25157 for a (maybe unrelated) reason so I'm filing this just for SA when the EDS muxing change goes back in.