Fix issues with failed ReleaseQueryBackends rpc

Currently, if the admission control service is being used, the coordinator will attempt the ReleaseQueryBackends rpc up to 3 times before giving up. If a ReleaseQueryBackends rpc fails it can lead to resources not being released properly. If a subsequent ReleaseQuery rpc succeeds, we'll hit a DCHECK as AdmissionController::ReleaseQuery() assumes that all backends have already been released.

We should clean this up and ensure that resources are always released properly.