bug: prevent java.util.ConcurrentModificationException in Transaction (#120)

- copy context set to work on individual entity
- updated demo documentation

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## References

#119 

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Author: dirkbolte

demo/README.md

@@ -89,11 +89,31 @@ cd wiremock-state-extension
 
 ### Step 2: Build the Project
 
+From directory of this README:
+
+```shell
+cd ..
 ./gradlew build
+```
+
+### Step 3: download wiremock standalone
+
+From directory of this README:
+
+```shell
+cd ..
+curl -o build/libs/wiremock-standalone-3.4.1.jar https://repo1.maven.org/maven2/org/wiremock/wiremock-standalone/3.4.1/wiremock-standalone-3.4.1.jar
+```
+
+### Step 4: Start WireMock with the State Extension
 
-### Start WireMock with the State Extension
+From directory of this README:
+
+```shell
+cd ..
+java -cp build/libs/wiremock-state-extension-standalone-0.6.0-SNAPSHOT.jar:build/libs/wiremock-standalone-3.4.1.jar wiremock.Run --verbose --global-response-templating --root-dir demo/stubs
+```
 
-java -jar build/libs/wiremock-standalone-*.jar --extensions="com.github.tomakehurst.wiremock.extension.StateExtension"
 This command starts WireMock with the State Extension enabled.
 
 ### Access the Demo

demo/docker_container_test.http

@@ -0,0 +1,8 @@
+### empty get
+
+POST http://localhost:8080//api/v1/todolist
+
+{
+  "title": "a",
+  "description": "as"
+}

src/main/java/org/wiremock/extensions/state/extensions/TransactionEventListener.java

@@ -54,6 +54,5 @@ public void afterComplete(ServeEvent serveEvent, Parameters parameters) {
         String requestId = serveEvent.getId().toString();
         var contextNames = transactionManager.getContextNamesByRequestId(requestId);
         contextNames.forEach((contextName) -> transactionManager.deleteTransaction(requestId, contextName));
-
     }
 }

src/main/java/org/wiremock/extensions/state/internal/TransactionManager.java

@@ -19,6 +19,7 @@
 import org.wiremock.extensions.state.internal.model.Transaction;
 
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 import java.util.function.Consumer;
@@ -59,7 +60,7 @@ public Set<String> getContextNamesByRequestId(String requestId) {
         var transactionKey = createTransactionKey(requestId);
         synchronized (store) {
             @SuppressWarnings("unchecked") var requestTransactions = store.get(transactionKey).map(it -> (Map<String, Transaction>) it).orElse(new HashMap<>());
-            return requestTransactions.keySet();
+            return new HashSet<>(requestTransactions.keySet());
         }
     }