@Override
  public boolean isSignalMemTable() {
    return true;
  }
Only used in sync flush and async close to start a flush task This memtable is not managed by
MemTablePool and does not store any data.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/flush/NotifyFlushMemTable.java