LAS VEGAS — Topic No. 1 for Rockies fans this winter is not about who’s on first in 2019. Nor is it about whether the club will add a power bat to wake up its sleepy offense. Those issues, important as they are, are secondary. The fate of all-star third baseman Nolan Arenado, who’s entering the final year of his contract, trumps everything, at least in the eyes of many fans.