feat : handle thread safe issue for getCurrentFutures (#1843)

### 📝 Description
improve thread safe issue for getCurrentFutures

current implementation get the values form cachemap for dataloaders.

And make it flatten. But when we get the values it's just iterator, not
all the elements is obtained.

So when we run flatten, it traverse the each iterator, and it make
```ArrayIndexOutOfBoundException```

Following is error message

```
aused by: java.lang.ArrayIndexOutOfBoundsException: Index 279 out of bounds for length 279
	at java.base/java.util.HashMap.valuesToArray(HashMap.java:973)
	at java.base/java.util.HashMap$Values.toArray(HashMap.java:1050)
	at java.base/java.util.ArrayList.addAll(ArrayList.java:670)
	at kotlin.collections.CollectionsKt__MutableCollectionsKt.addAll(MutableCollections.kt:116)
	at kotlin.collections.CollectionsKt__IterablesKt.flatten(Iterables.kt:49)
	at com.expediagroup.graphql.dataloader.KotlinDataLoaderRegistry.getCurrentFutures(KotlinDataLoaderRegistry.kt:67)
	at com.expediagroup.graphql.dataloader.KotlinDataLoaderRegistry.dispatchAll(KotlinDataLoaderRegistry.kt:78)
	at graphql.execution.instrumentation.dataloader.FieldLevelTrackingApproach.dispatch(FieldLevelTrackingApproach.java:242)
	at graphql.execution.instrumentation.dataloader.FieldLevelTrackingApproach$1.onFieldValuesInfo(FieldLevelTrackingApproach.java:150)
	at graphql.execution.AsyncExecutionStrategy.lambda$execute$1(AsyncExecutionStrategy.java:72)
	at java.base/java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:863)
	... 50 common frames omitted
```

### 🔗 Related Issues
#1837

Author: younseunghyun

executions/graphql-kotlin-dataloader/src/main/kotlin/com/expediagroup/graphql/dataloader/KotlinDataLoaderRegistry.kt

@@ -61,11 +61,11 @@ class KotlinDataLoaderRegistry(
      * @return list of current completable futures.
      */
     fun getCurrentFutures(): List<CompletableFuture<*>> =
-        synchronized(registry.dataLoaders) {
-            registry.dataLoaders.map { dataLoader ->
-                dataLoader.cacheMap.all
-            }.flatten()
-        }
+        registry.dataLoaders.map { dataLoader ->
+            synchronized(dataLoader) {
+                dataLoader.cacheMap.all.toList()
+            }
+        }.flatten()
 
     /**
      * This will invoke [DataLoader.dispatch] on each of the registered [DataLoader]s,