Avoid task rescheduling when a node turns unhealthy

This is similar to MAPREDUCE-6119. Sometimes reacting to a node update event can cause more harm than good. For example, an UNHEALTHY node may be able to shuffle just fine. Therefore obsoleting the output of tasks that ran on that node and re-running them simply adds more overhead to the job with no benefit. It would be nice to be able to configure Tez to ignore node update events if desired.