can lead to a lot of unnecessary cacheline ping-pong. Instead
GetSnapshotData updates approximate thresholds (one that guarantees that all
deleted rows older than it can be removed, another determining that deleted
-rows newer than it can not be removed). GlobalVisTest* uses those threshold
+rows newer than it can not be removed). GlobalVisTest* uses those thresholds
to make invisibility decision, falling back to ComputeXidHorizons if
necessary.