@Override
  public MemStoreSize getMemStoreSize() {
    return new MemStoreSize(this.dataSize, this.heapSize, this.offHeapSize, this.cellsCount);
  }
Accounting of current heap and data sizes. <em>NOT THREAD SAFE</em>. Use in a 'local' context
only where just a single-thread is updating. No concurrency! Used, for example, when summing all
Cells in a single batch where result is then applied to the Store.
@see ThreadSafeMemStoreSizing
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/NonThreadSafeMemStoreSizing.java