Arenado strongly in Rockies' mix

  Nolan Arenado is that rarest of rookies – one who might make an Opening Day roster without regard to service-time considerations.   The Colorado Rockies likely will wait another 7-10 days before deciding whether Arenado will be their starting third baseman. But the team is giving the idea serious consideration, according to major-league sources   “It’s a tough call,” one club official said.   Chris Nelson, the team’s incumbent third baseman, batted .344 with an .881 OPS in 180 at-bats after the All-Star Game. But Nelson, 27, lacks an established track record, and the Rockies might simply decide it is Arenado’s time.   For Arenado and surging rookies such as Boston outfielder Jackie Bradley Jr. and Minnesota outfielder Aaron Hicks, the final weeks of spring training will be critical as they face pitchers sharpening their repertoires in advance of Opening Day.   Teams generally hold off on promoting rookies immediately in order to delay their eligibility for salary arbitration and