Speed up DistWriteSkewTest and its subclasses

DistWriteSkewTest is annotated with @CleanupAfterMethod, so the cluster is recreated for each one of the test methods. Since there are a lot of test methods inherited from AbstractClusteredWriteSkewTest, the repeated cluster startup and shutdown add a lot of overhead.

Since the test doesn't start or stop nodes, it should be possible to start the cluster only once for the whole class, eventually using different key names if leftover keys turn out to be a problem.

The annotation is also used for ReplWriteSkewTest, DistL1WriteSkewTest, and DistTotalOrderWriteSkewTest. It should be possible to reuse the cluster for all of them.