Finish sftp refactoring. Test suite passing.

* bzrlib/transport/sftp.py:
(clear_connection_cache): Deprecated.
(_sftp_connect, _sftp_connect_uncached): Deleted.
(SFTPTransport.__init__): Simplified.
(SFTPTransport._create_connection): New method. Copied from
_sftp_connect_uncached
(SFTPTransport._get_sftp): New method. Ensures that the connection
is established.
(SFTPTransport.clone): Deleted.
(SFTPTransport.has, SFTPTransport.get, SFTPTransport.readv,
SFTPTransport._put,
SFTPTransport._put_non_atomic_helper._open_and_write_file,
SFTPTransport._mkdir, SFTPTransport.append_file,
SFTPTransport.rename, SFTPTransport._rename_and_overwrite,
SFTPTransport.delete, SFTPTransport.rmdir, SFTPTransport.stat):
Use _get_sftp.

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test_connection_error): Simplified now that sftp
does not connection on construction.

* bzrlib/tests/test_sftp_transport.py:
(SFTPLockTests.test_sftp_locks): Delete test_multiple_connections.
(FakeSFTPTransport): Deleted.
(SFTPNonServerTest.test_parse_url_with_home_dir,
SFTPNonServerTest.test_relpath,
SSHVendorBadConnection.test_bad_connection_paramiko): Delete the
from_transport parameter as it's not needed anymore.
(SFTPLatencyKnob.test_latency_knob_slows_transport,
SFTPLatencyKnob.test_default): Force connection by issuing a
request.