Zookeeper free workload generator

Closes #146 (closed).

Still missing:

• Adapt Theodolite execution Kubernetes resources and Python scripts
• Adapt Docker configurations (to fix #106 (closed))

benchmarks/workload-generator-commons/src/main/java/theodolite/commons/workloadgeneration/functions/MessageGenerator.java

 package theodolite.commons.workloadgeneration.functions;
 
+/**
+ * Interface representing a message generator, which sends messages for given keys to some
+ * destination.
+ */
 @FunctionalInterface
 public interface MessageGenerator {