@RestrictedApi(explanation = "Should only be called in tests", link = "",
      allowedOnPath = ".*/src/test/.*")
  IntHashSet getFailedReplicas() {
    synchronized (entries) {
      return this.failedReplicas;
    }
  }
Make sure that we have finished all the replicating requests.
<p/>
After returning, we can make sure there will be no new replicating requests to secondary
replicas.
<p/>
This is used to keep the replicating order the same with the WAL edit order when writing.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/regionreplication/RegionReplicationSink.java