private <K1, K2, V> List<V> addToHashMultiMap(Map<K1, Map<K2, List<V>>> map, K1 key1, K2 key2,
    V value) {
    Map<K2, List<V>> innerMap = map.computeIfAbsent(key1, k -> new HashMap<>());
    List<V> values = innerMap.computeIfAbsent(key2, k -> new ArrayList<>());
    values.add(value);
    return values;
  }
Simple helper to a map from key to (a list of) values TODO: Make a general utility method
@return the list of values corresponding to key1 and key2
hbase-server/src/main/java/org/apache/hadoop/hbase/replication/regionserver/ReplicationSink.java