The smart server leaks one thread (daemonic) for each connection.

* bzrlib/tests/test_smart_transport.py:
(SmartTCPTests.start_server): Use stop_server for consistency.
(SmartTCPTests.stop_server): Rename from tearDownServer for
consistency.
(TestServerSocketUsage.test_server_closes_listening_sock_on_shutdown_after_request):
Keep track of the server URL explicitly.

* bzrlib/smart/server.py (SmartTCPServer.serve_conn):
Mark the origin of the leak.