Run tests in parallel

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 brings "mvn verify" times down to 20 minutes on Travis. On fast multi-core machines you can get down to 7 minutes. :smile: