Improved memory management

From now on, each configuration should be ran in completely new classloader and everything loaded by previous classloader should be dropped. This needs some discipline: don't store in slave state anything that is plugin-specific, only instances of the core/extension classes.
If you still got OOME in PermGen space (when using many configurations), it's likely that something is still referenced (e.g. as MBeans) or that some threads haven't been terminated (although ScenarioCleanupStage calls Thread.stop() on all threads that outlive the configuration), we should create heap dump and inspect it to find the link.