Login callback behaviour needs to be reviewed

[yasserf]

We need to review the login behaviour in deepstream in general. Having a login callback being called more than once should be invalid behaviour

[jdmnd]

proposed new method signature:

login(auth_data
login_callback // (success) => ..., 
client_data_received_callback // (client_data) => ...
)

login_callback is called exactly once, when auth succeeds or fails. it is destructed thereafter
client_data_received_callback is called once if auth succeeds and again whenever there is a transition AUTHENTICATING -> OPEN