This removes the explicit Sync things from the thread wraper since
the functionality is almost exactly provided by the backend.
Thread encapsulates a lumiera thread handle now, but this is strictly
optional and might be dropped on the floor (using a temporary as thread).
Thread has a sync() function which allows user controlled syncronous
startup:
Thread("foo", myoperation).sync();
will startup myoperation and only return from the ctor after myoperation
called a matching lumiera_thread_sync().
The related tests need to initialize/destroy the threadpool accordingly
