backend: get_redis_logger: skip log entries bellow log_level
    
    The RedisLogHandler already reads and respects log_level option from
    configuration (by default INFO), so we don't actually waste the space in
    log files.  But it is even useless to overwhelm redis server with
    messages which are eventually not going to be printed to log file.  So
    from now, the "level" is respected also on log entry "sender" side (and
    I personally think that we could respect the level _only_ on sender
    side, and let the RedisLogHandler dump everything is received, ... maybe
    next PR?).
    
    This is reaction for magically disappearing log entries upon Worker
    failure (exceptions should be logged, but are not).  I suppose that the
    redis server isn't ready to process all the log entries, and it leads to
    obscure system states.
    
    Merges: #1065