This class represents a split policy which makes the split decision based
on how busy a region is. The metric that is used here is the fraction of
total write requests that are blocked due to high memstore utilization.
This fractional rate is calculated over a running window of
"hbase.busy.policy.aggWindow" milliseconds. The rate is a time-weighted
aggregated average of the rate in the current window and the
true average rate in the previous window.