RocksDB 4.1 发布，更新内容如下：
Added single delete operation as a more efficient way to delete keys that have not been overwritten.
Added experimental AddFile() to DB interface that allow users to add files created by SstFileWriter into an empty Database, see include/rocksdb/sst_file_writer.h and DB::AddFile() for more info.
Added support for opening SST files with .ldb suffix which enables opening LevelDB databases.
CompactionFilter now supports filtering of merge operands and merge results.
公共 API 更新
Added SingleDelete() to the DB interface.
Added AddFile() to DB interface.
Added SstFileWriter class.
CompactionFilter has a new method FilterMergeOperand() that RocksDB applies to every merge operand during compaction to decide whether to filter the operand.
We removed CompactionFilterV2 interfaces from include/rocksdb/compaction_filter.h. The functionality was deprecated already in version 3.13.