Consolidate VACUUM xid cutoff logic.
Push the logic for determining whether or not a VACUUM operation will be
aggressive down into vacuum_set_xid_limits(). This makes the function's
signature significantly simpler, and seems clearer overall.
Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkymFbz6D_vL+jmqSn_5q1wsFvFrE+37yLgL_Rkfd6Gzg@mail.gmail.com