protected synchronized void addSubProcedure(Procedure<TEnvironment> proc) {
    if (!proc.hasParent()) {
      return;
    }
    if (subprocs == null) {
      subprocs = new HashSet<>();
    }
    subprocs.add(proc);
  }
Called by the ProcedureExecutor after the procedure step is completed, to add the step to the
rollback list (or procedure stack).
<p>
Return whether we successfully added the rollback step. If the root procedure has already
crossed the PONR, we do not need to maintain the rollback step,
hbase-procedure/src/main/java/org/apache/hadoop/hbase/procedure2/RootProcedureState.java