This closes #98

This uses the forkCount parameter of surefire/failsafe. This also
changes the JVM memory size for the testing VMs so that more can fit
into memory.

For this to work, NepheleMiniCluster must recognize when it is run in
a parallel testing environment and change ports accordingly. Also
AbstractTestBase and TaskTestBase must prefix temp files with the class
name to prevent name clashes.

This closes #96

This closes #97

This closes #75

This closes #42

We now check whether the key fields specified for the delta iterations
match those specified when joining/coGrouping with the solution set.
This code is more or less copied from the new Java API.

The Scala API uses the Record API underneath so it is also covered.

- remove usages of commons-lang (we use only commons-lang3, as defined in flink-runtime/pom.xml)
- Upgrade maven failsafe to 2.17 so that tests also fail if the test errors (exceptions during execution)
- added new build profile "docs-and-source" that creates javadocs and source attachments. These are usually only needed when releasing (--> speed up the build)

This closes #90