Keep credentials used at connection creation for reconnection purposes.

* bzrlib/transport/__init__.py:
(ConnectedTransport._init_connection): New method. Keep related
code together.
(ConnectedTransport._set_connection): Add a credentials parameter.
(ConnectedTransport._get_credentials): New method.

* bzrlib/tests/test_transport.py:
(TestConnectedTransport.test_connection_sharing_propagate_credentials):
New test.