Solve issue in the server when shooting player not logged in

If a player not logged in the server was requested to be shot, the
server crashed with a std::out_of_range exception.
Health updates come only from the server

Previously we updated locally the player health with the damage
information. Now, the server is notified of the damage and it
updates the player, and the local player is updated when the
server sends back the player info.
BufferedPort<T> is now an attibute of the YarpNetworkManager

Changed yarp::os::BufferedPort<T> to be an attribute of the
YarpNetworkManager to improve consistency in the class.

The callback is still implemented on the NetworkManager, and
added later to the BufferedPort.