CompactorPipeline addCompactor(Compactor newCompactor) {
    compactor = new FallbackCompactor(compactor, newCompactor);
    return this;
  }
Runs different compactions based on the order in which compactors are added.<br>
Mainly used for fall back mechanism for Merge compaction.
ql/src/java/org/apache/hadoop/hive/ql/txn/compactor/CompactorPipeline.java