Remove Zookeeper dependency for workload generators

Currently our load generators use Zookeeper for coordination. Zookeeper provides built-in support for everything we need for coordination, but has the disadvantage that all state is manged by a separately deployed system and for restarting the load generators, we have to reset Zookeepers state.

We plan to avoid such an external system and let the individual load generators directly talk to each other. To achieve this, we will use Hazelcast as an embedded library in our load generators. Although Hazelcast might be a bit over-sized what our task, it seems that this is currently the easiest option. Later we could think about using Apache Ratis, which should be enough for our needs, but it is currently quite bad documented.

Will allow fixing #106 (closed).