@Override
  public void nodeChildrenChanged(String path) {
    if (
      path.equals(watcher.getZNodePaths().rsZNode) && !server.isAborted() && !server.isStopped()
    ) {
      executor.execute(this::refresh);
    }
  }
Upgrade to active master mode, where besides tracking the changes of region server set, we will
also started to add new region servers to ServerManager and also schedule SCP if a region
server dies. Starts the tracking of online RegionServers. All RSes will be tracked after this
method is called.
<p/>
In this method, we will also construct the region server sets in {@link ServerManager}. If a
region server is dead between the crash of the previous master instance and the start of the
current master instance, we will schedule a SCP for it. This is done in
{@link ServerManager#findDeadServersAndProcess(Set, Set)}, we call it here under the lock
protection to prevent concurrency issues with server expiration operation.
@param deadServersFromPE          the region servers which already have SCP associated.
@param liveServersBeforeRestart   the live region servers we recorded before master restarts.
@param splittingServersFromWALDir Servers whose WALs are being actively 'split'.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/RegionServerTracker.java