Reset configSelector on realChannel while entering IDLE state

[kkalin68]

ManagedChannel will stuck in IDLE state when xDS control plane doesn't have a resource anymore.
The scenario is following:

1. Channel is open for a xds resource.
2. XdsNameResolver subscribes to the resource on xDS control plane.
3. The resource is removed from xDS control plane for extended period of time (unhealthy for more than the idle timeout on Channel)
4. Channel enters into TRANSIENT_FAILURE
5. The Idle timeout triggers and Channel shutdowns XdsNameResolver and other resources. xDS watchers are removed.
6. The resource comes back online on xDS control plane.
7. A new GRPC call is executed targeting the channel.
8. The channel stays in IDLE state and reports: "io.grpc.StatusRuntimeException: UNAVAILABLE: LDS resource xxxx does not exist nodeID: yyyy" because realChannel.configSelector still points to old state.

public <ReqT, RespT> ClientCall<ReqT, RespT> newCall(
        MethodDescriptor<ReqT, RespT> method, CallOptions callOptions) {
      if (configSelector.get() != INITIAL_PENDING_SELECTOR) {
        return newClientCall(method, callOptions);
      }
...

[linux-foundation-easycla]

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• ✅ login: kkalin68 / name: Konstantin Kalin (f09b7d9)