don't log a line every time the checks detects the condition.

only log when the notification wasn't there before so we don't flood the log unnecessarily
also mention the threshold value in the log line
increase the level from info to warn because it really is a warning
cycling the deflector index could fail to update the index range for newly created index

if the deflector target index was empty it would get no corresponding index range assigned.
now for the deflector target graylog2 will always properly create an index range entry

fixes Graylog2/graylog2-web-interface#656