Refactored dht_put_item to dht_put_immutable_item in session_impl since the get operations are not overloaded (consistency).
Added allow_threading_guard to add_dht_node python binding.
Making dht_put_item in session returns a zeroed hash if TORRENT_DISABLE_DHT (avoid unnecessary calculation and it's more consistent).
Added missing DHT operations to the python binding.
Added missing allow_threading_guard to add_dht_node in the python binding.