[backend] fix traceback for non-serializable log message
    
    In some cases we happen to log (through redis logger) objects
    which are not json-serializable (by default).  This caused
    ugly exceptions before, e.g.
    
       TypeError: Object of type 'Thread' is not JSON serializable
    
    So let's provide some (even if not really nice) always-ready
    serialization default so we never loose the whole log message
    because of the traceback.