Create a new, empty, ConnectedTransport class.
* bzrlib/transport/sftp.py:
(SFTPUrlHandling): Based on ConnectedTransport.
* bzrlib/transport/remote.py:
(RemoteTransport): Based on ConnectedTransport.
* bzrlib/transport/http/__init__.py:
(HttpTransportBase): Based on ConnectedTransport.
* bzrlib/transport/ftp.py:
(FtpTransport): Based on ConnectedTransport.
* bzrlib/transport/__init__.py:
(ConnectedTransport): New class.