Various stability improvements & Cleanup

[johannesduesing]

This PR essentially implements the improvements we found when optimizing our artifact during the evaluation. This includes:

• Closing all Input Streams / URL connections that we open
• Remove main method (and corresponding plugin in pom) - only for the evaluation we need an executable JAR
• Improve relevant error logging, remove logging of FileNotFoundExceptions as they do not constitute an error
• Use AtomicInteger where relevant to improve multithreaded behavior
• Enforce HTTP timeouts
• Recover from connection interruptions in iterator
• Some minor code warnings cleaned up