public byte[] getStartKey() {
        return startKey;
    }
Region is the most basic kv data unit.  Each region has a left-closed
right-open interval range.  The region is responsible for the crud
request in the range.  Each region is a raft group, which is distributed
on different store nodes. Each region copy is called a region peer.
As the data volume of the region reaches the threshold, it will trigger
split. In fact, it is only the metadata change. This action is very fast.
The two regions just after the split are still on the original store node.
The PD periodically checks the number of regions in each store node, and
the load situation. The raft snapshot is used to migrate the region between
the store nodes to ensure the load balance of the cluster.

@author jiachun.fjc
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/metadata/Region.java