### Description

`SearchResponse.Clusters` currently uses a `HashMap<String, AtomicReference>` for storing Cluster objects. The AtomicReference CAS swaps are done in a do/while loop in most cases (e.g., in `CCSSingleCoordinatorSearchProgressListener`).

This can be replaced with a `ConcurrentHashMap` in conjunction with either `compute()` method or the `replace(key, assumed_previous_value, value)` method for atomically swapping in a new Cluster object with a CAS operation, assuring that the updates don't lose data from competing threads doing separate Cluster object updates.

See: https://github.com/elastic/elasticsearch/pull/98457/files#r1310467586 
x-pack/plugin/async-search/src/internalClusterTest/java/org/elasticsearch/xpack/search/CrossClusterAsyncSearchIT.java
}
    protected AsyncSearchResponse submitAsyncSearch(SubmitAsyncSearchRequest request) throws ExecutionException, InterruptedException {
        return client(LOCAL_CLUSTER).execute(SubmitAsyncSearchAction.INSTANCE, request).get();
    }
    protected AsyncSearchResponse getAsyncSearch(String id) throws ExecutionException, InterruptedException {
        return client(LOCAL_CLUSTER).execute(GetAsyncSearchAction.INSTANCE, new GetAsyncResultRequest(id)).get();
    }
    protected AsyncStatusResponse getAsyncStatus(String id) throws ExecutionException, InterruptedException {
        return client(LOCAL_CLUSTER).execute(GetAsyncStatusAction.INSTANCE, new GetAsyncStatusRequest(id)).get();
    }
    protected AcknowledgedResponse deleteAsyncSearch(String id) throws ExecutionException, InterruptedException {
        return client().execute(DeleteAsyncResultAction.INSTANCE, new DeleteAsyncResultRequest(id)).get();
    }
    private Map<String, Object> setupTwoClusters() {
        String localIndex = "demo";
        int numShardsLocal = randomIntBetween(2, 12);
        Settings localSettings = indexSettings(numShardsLocal, randomIntBetween(0, 1)).build();
        assertAcked(
            client(LOCAL_CLUSTER).admin()
                .indices()
                .prepareCreate(localIndex)
                .setSettings(localSettings)
                .setMapping("@timestamp", "type=date", "f", "type=text")
        );
        indexDocs(client(LOCAL_CLUSTER), localIndex);
        String remoteIndex = "prod";
        int numShardsRemote = randomIntBetween(2, 12);
        final InternalTestCluster remoteCluster = cluster(REMOTE_CLUSTER);
        remoteCluster.ensureAtLeastNumDataNodes(randomIntBetween(1, 3));
        final Settings.Builder remoteSettings = Settings.builder();