The new name conveys the effect better, is more consistent with similar
functions ReadNextMultiXactId(), ReadNextFullTransactionId(), and
matches the name of the variable that it reads.
Reported-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzmVR4SakBXQUdhhPpMf1aYvZCnna5%3DHKa7DAgEmBAg%2B8g%40mail.gmail.com