Move thread logging from REEFEnvironment to REEFLauncher

We need to log all threads that are still running at the end of REEF process. With the introduction of the Unmanaged AM and REEF-as-a-library modes, closure of the REEFEnvironment is not always the end of the REEF process. Therefore, we need to move the invocation of thread logging from REEFEnvironment.close() to the end of REEFLauncher.main() call.