Use the newest to-be-frozen xid as the conflict horizon for freezing

Use the newest to-be-frozen xid as the conflict horizon for freezing

Previously WAL records that froze tuples used OldestXmin as the snapshot
conflict horizon, or the visibility cutoff if the page would become
all-frozen. Both are newer than (or equal to) the newst XID actually
frozen on the page.

Track the newest XID that will be frozen and use that as the snapshot
conflict horizon instead. This yields an older horizon resulting in
fewer query cancellations on standbys.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAAKRu_bbaUV8OUjAfVa_iALgKnTSfB4gO3jnkfpcFgrxEpSGJQ%40mail.gmail.com