@Override
  public String toString() {
    return "TableProcedureWaitingQueue [enqueuedProc=" + enqueuedProc + ", queue=" + queue + "]";
  }
Return the next procedure which can be enqueued to ProcedureScheduler.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/TableProcedureWaitingQueue.java