Trace reason when session gets disconnected

When a session is disconnected (Session.disconnect()), we just get a "<session> disconnected" trace. In some case, the disconnection is a normal behavior (e.g. after a logout), in some other it is not (e.g. socket failure). It is therefore difficult to troubleshot FIX connectors with this lack of information.
Add a "reason" string parameter to the disconnect method and log it.