@Override
  protected long heapSize() {
    // Need to consider heapOverhead of all segments in pipeline and active
    long h = getActive().getHeapSize();
    for (Segment segment : this.pipeline.getSegments()) {
      h += segment.getHeapSize();
    }
    return h;
  }
Push the current active memstore segment into the pipeline and create a snapshot of the tail of
current compaction pipeline Snapshot must be cleared by call to {@link #clearSnapshot}.
{@link #clearSnapshot(long)}.
@return {@link MemStoreSnapshot}
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompactingMemStore.java