void findDeadServersAndProcess(Set<ServerName> deadServersFromPE,
    Set<ServerName> liveServersFromWALDir) {
    deadServersFromPE.forEach(deadservers::putIfAbsent);
    liveServersFromWALDir.stream().filter(sn -> !onlineServers.containsKey(sn))
      .forEach(this::expireServer);
  }
Find out the region servers crashed between the crash of the previous master instance and the
current master instance and schedule SCP for them.
<p/>
Since the {@code RegionServerTracker} has already helped us to construct the online servers set
by scanning zookeeper, now we can compare the online servers with {@code liveServersFromWALDir}
to find out whether there are servers which are already dead.
<p/>
Must be called inside the initialization method of {@code RegionServerTracker} to avoid
concurrency issue.
@param deadServersFromPE     the region servers which already have a SCP associated.
@param liveServersFromWALDir the live region servers from wal directory.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java