public boolean isEmpty() {
    return readOnlyCopy.isEmpty();
  }
If the caller holds the current version, go over the the pipeline and try to flatten each
segment. Flattening is replacing the ConcurrentSkipListMap based CellSet to CellArrayMap based.
Flattening of the segment that initially is not based on ConcurrentSkipListMap has no effect.
Return after one segment is successfully flatten.
@return true iff a segment was successfully flattened
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompactionPipeline.java