Ensure that all transports close their underlying connection.

* bzrlib/transport/sftp.py:
(SFTPTransport.disconnect): Close the sftp connection.

* bzrlib/transport/remote.py:
(RemoteTransport.disconnect): The medium can be None.

* bzrlib/transport/http/_urllib.py:
(HttpTransport_urllib.disconnect): Close the http connection.

* bzrlib/transport/http/_pycurl.py:
(PyCurlTransport.disconnect): Close the Curl object.

* bzrlib/transport/http/__init__.py:
(SmartClientHTTPMedium.disconnect): Disconnect the backing http
transport.

* bzrlib/transport/gio_transport.py:
(GioTransport.disconnect): Hmm, nothing obvious here.

* bzrlib/transport/ftp/__init__.py:
(FtpTransport.disconnect): Close the ftplib connection.

* bzrlib/transport/__init__.py:
(Transport.disconnect): A do-nothing implementation by default.
(ConnectedTransport.disconnect): A new required method.

* bzrlib/tests/test_smart_transport.py:
(TestSmartTCPServer.test_get_error_unexpected)
(SmartTCPTests.start_server): Revert to SmartServer waiting for a
better way to capture the connections.

* bzrlib/tests/per_transport.py:
(TransportTests.test_connection_sharing): 'None' is better than
'object()' for a dummy connection.

* bzrlib/tests/__init__.py:
(TestCase._check_leaked_threads): Don't fail at first leak or all
subsequent tests will fail too. A more elaborate scheme would be
to use threading.enumerate() but that's a big hammer with little
benefits as long as there exist "valid" leaks (like the smart
server).
(TestCaseWithMemoryTransport.setUp): Make sure we call
transport.disconnect() for all created transports.