public RegionMoverBuilder ack(boolean ack) {
      this.ack = ack;
      return this;
    }
Set ack/noAck mode.
<p>
In ack mode regions are acknowledged before and after moving and the move is retried
hbase.move.retries.max times, if unsuccessful we quit with exit code 1.No Ack mode is a best
effort mode,each region movement is tried once.This can be used during graceful shutdown as
even if we have a stuck region,upon shutdown it'll be reassigned anyway.
<p>
@return RegionMoverBuilder object
hbase-server/src/main/java/org/apache/hadoop/hbase/util/RegionMover.java