protected Action getMergingAction() {
    return Action.MERGE;
  }
Types of actions to be done on the pipeline upon MemStoreCompaction invocation. Note that every
value covers the previous ones, i.e. if MERGE is the action it implies that the youngest
segment is going to be flatten anyway.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreCompactionStrategy.java