Upgrade Beam version

Depends on !187 (merged).

Fix #304 (closed).

Open issues:

• null can (probably) no longer be used as tombstone in UC4 as reported by spotbugs
• test all implementations (see below)
  • UC1 / Flink
  • UC1 / Samza
  • UC2 / Flink
  • UC2 / Samza
  • UC3 / Flink
  • UC3 / Samza
  • UC4 / Flink
  • UC4 / Samza