fix ConcurrentModificationException when completing invocations

Author: zhongkechen

sdk/src/main/java/software/amazon/lambda/durable/execution/ExecutionManager.java

@@ -5,14 +5,14 @@
 import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.stream.Collectors;
@@ -59,7 +59,7 @@ public class ExecutionManager implements AutoCloseable {
     private final DurableConfig durableConfig;
 
     // ===== Thread Coordination =====
-    private final Map<String, BaseDurableOperation> registeredOperations = Collections.synchronizedMap(new HashMap<>());
+    private final Map<String, BaseDurableOperation> registeredOperations = new ConcurrentHashMap<>();
     private final Set<String> activeThreads = Collections.synchronizedSet(new HashSet<>());
     private static final ThreadLocal<ThreadContext> currentThreadContext = new ThreadLocal<>();
     private final CompletableFuture<Void> executionExceptionFuture = new CompletableFuture<>();