Refactor SFTPTransport. Test suite passes.
* bzrlib/transport/sftp.py:
(SFTPUrlHandling): Simplified.
(SFTPTransport._remote_path): Deleted. SFTPUrlHandling now has a
simpler version.
* bzrlib/tests/test_sftp_transport.py:
Refactor the tests, most of them have gone to TestConnectedTransport.
(SFTPNonServerTest.test_parse_url_with_home_dir): Renamed from
test_parse_url and simplified to keep only specific tests.