Migrate Flink benchmark implementation

This merge request will fix #20 (closed). It is based on the existing implementation in the apache-flink branch but has to tackle the following requirements:

• Migrate the individual benchmarks with their new enumeration
  • Migrate UC1
  • Migrate UC2 (f.k.a. UC3)
  • Migrate UC3 (f.k.a. UC4)
  • Migrate UC4 (f.k.a. UC2)
    • Migrate to latest architecture including dedicated feedback loop
      • Moved to #173
• Migrate to Titan Control Center Commons 0.1.0
• Fix code quality issues
• Support Docker builds (already available in apache-flink)
• Build and push Docker images in CI pipeline
• Update Docker Compose files in docker-test
• Enhance Gradle configuration sharing (see #107 (closed), might be tackled in a follow up MR)
  • Moved to !95 (merged)
• Clean up Gradle build files (see #59, might be tackled in a follow up MR)
  • Moved to #59
• Rename Gradle subprojects. Envisaged naming is already available in settings.gradle files as comments)
  • Moved to #174 (closed)
• Provide corresponding Kubernetes benchmark definitions (might be tackled in a follow up ticket)
  • Moved to #176 (closed)

theodolite-benchmarks/uc2-application-flink/src/main/java/theodolite/uc2/application/StatsProcessWindowFunction.java

@@ -6,6 +6,10 @@ import org.apache.flink.streaming.api.functions.windowing.ProcessWindowFunction;
 import org.apache.flink.streaming.api.windowing.windows.TimeWindow;
 import org.apache.flink.util.Collector;
 
+/**
+ * A {@link ProcessWindowFunction} that forwards a computed {@link Stats} object along with its
+ * associated key.
+ */
 public class StatsProcessWindowFunction
     extends ProcessWindowFunction<Stats, Tuple2<String, Stats>, String, TimeWindow> {