ExecutorFactory: switch to CopyOnWriteArrayList for executionListeners

This would allow adding/removing executionListeners while in the middle of an execution listener. The current
version of the DAP adapter relies on being able to attach itself during the "about to execute" event.

plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/ExecutorFactory.java

@@ -13,6 +13,8 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.concurrent.CopyOnWriteArrayList;
+
 import org.eclipse.epsilon.common.function.BaseDelegate;
 import org.eclipse.epsilon.common.module.ModuleElement;
 import org.eclipse.epsilon.eol.EolModule;
@@ -22,7 +24,10 @@
 import org.eclipse.epsilon.eol.exceptions.EolRuntimeException;
 import org.eclipse.epsilon.eol.exceptions.flowcontrol.EolTerminationException;
 import org.eclipse.epsilon.eol.execute.context.IEolContext;
-import org.eclipse.epsilon.eol.execute.control.*;
+import org.eclipse.epsilon.eol.execute.control.DefaultExecutionController;
+import org.eclipse.epsilon.eol.execute.control.ExecutionController;
+import org.eclipse.epsilon.eol.execute.control.ExecutionProfiler;
+import org.eclipse.epsilon.eol.execute.control.IExecutionListener;
 
 /**
  * This class is not thread-safe.
@@ -72,7 +77,7 @@ public final StackTraceManager getStackTraceManager() {
 
 	public void addExecutionListener(IExecutionListener listener) {
 		if (executionListeners == null) {
-			executionListeners = new ArrayList<>(1);
+			executionListeners = new CopyOnWriteArrayList<>();
 		}
 		executionListeners.add(listener);
 	}