Occasional ConcurrentModificationException making test runs last forever

[S1artie]

This appears to be a new problem since Integrity 0.16. Occasionally (and until now only observed on build servers) some tests seem to run into the following ConcurrentModificationException:

FORK '': Exception in thread "Integrity - Endpoint Output Processor" com.esotericsoftware.kryo.KryoException: java.util.ConcurrentModificationException
	FORK '': Serialization trace:
	FORK '': attributes (de.gebit.integrity.remoting.entities.setlist.SetListEntry)
	FORK '': updatedEntries (de.gebit.integrity.remoting.transport.messages.SetListUpdateMessage)
	FORK '': 	at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:101)
	FORK '': 	at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:505)
	FORK '': 	at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:651)
	FORK '': 	at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:362)
	FORK '': 	at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:303)
	FORK '': 	at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:575)
	FORK '': 	at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:80)
	FORK '': 	at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:505)
	FORK '': 	at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:651)
	FORK '': 	at de.gebit.integrity.remoting.transport.Endpoint$EndpointOutputProcessor.run(Endpoint.java:376)
	FORK '': Caused by: java.util.ConcurrentModificationException
	FORK '': 	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
	FORK '': 	at java.util.HashMap$EntryIterator.next(HashMap.java:1463)
	FORK '': 	at java.util.HashMap$EntryIterator.next(HashMap.java:1461)
	FORK '': 	at com.esotericsoftware.kryo.serializers.MapSerializer.write(MapSerializer.java:99)
	FORK '': 	at com.esotericsoftware.kryo.serializers.MapSerializer.write(MapSerializer.java:39)
	FORK '': 	at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:575)
	FORK '': 	at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:80)

This, as a result, seems to prevent the entire test scenario from exiting, resulting in the fork as well as the master running forever (and idling).