private List<RegionInfo> getSystemTables(ServerName serverName) {
    ServerStateNode serverNode = regionStates.getServerNode(serverName);
    if (serverNode == null) {
      return Collections.emptyList();
    }
    return serverNode.getSystemRegionInfoList();
  }
Start a new thread to check if there are region servers whose versions are higher than others.
If so, move all system table regions to RS with the highest version to keep compatibility. The
reason is, RS in new version may not be able to access RS in old version when there are some
incompatible changes.
<p>
This method is called when a new RegionServer is added to cluster only.
</p>
hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java