code: public Map<String, Map<String, List<String>>> getColumnDeletesMap() {
          Map<String, Map<String, List<String>>> storeMap = new HashMap<>();
          for(ColumnDelete mutation: getColumnDeletes()) {
              String storeName = mutation.getStoreName();
              String rowKey = mutation.getRowKey();
              String column = mutation.getColumnName();
              Map<String, List<String>> rowMap = storeMap.get(storeName);
              if(rowMap == null) {
                  rowMap = new HashMap<>();
                  storeMap.put(storeName, rowMap);
              }
              List<String> columnsList = rowMap.get(rowKey);
              if(columnsList == null) {
                  columnsList = new ArrayList<>();
                  rowMap.put(rowKey, columnsList);
              }
              columnsList.add(column);
          }
          return storeMap;
      }
docs: Get the map of the column deletes as storeName -> rowKey -> list of column names