Don't wait for thread to exit in __del__

This can hit a deadlock waiting for the thread to exit.  We don't
need to wait; it will die when it can, or get terminated when the
interpreter exits.

nilmdb/utils/serializer.py

@@ -118,8 +118,8 @@ def serializer_proxy(obj_or_type):
 
         def __del__(self):
             try:
+                # Signal thread to exit, but don't wait for it.
                 self.__call_queue.put((None, None, None, None))
-                self.__thread.join()
             except: # pragma: no cover
                 pass