Fix issues with timestamp json dump
    
    Arrow.timestamp() is a method not property in newer arrow releases.
    
    We will use float_timestamp instead, which is available even in older versions.
    
    See https://arrow.readthedocs.io/en/latest/#arrow.arrow.Arrow.timestamp
    
    Signed-off-by: Michal Konečný <mkonecny@redhat.com>