Feat: support copy file

[KKould]

If the snapshot is on the same filesystem as the original database, the SST files will be hard-linked, otherwise SST files will be copied

https://rocksdb.org/blog/2015/11/10/use-checkpoints-for-efficient-snapshots.html

CheckPoint will "copy" the WAL corresponding to MemTable and the related SSTable in Version. However, tonbo supports multiple FileSystems based on fusio. I think fusio can copy file to solve this problem.

• ref issue: Checkpoint support tonbo#209
• ref api: copy & hard_link

[ethe]

fusio::Fs should provide a unified "copy file" semantic, "try hard-linked first otherwise copy" may be a good choice. To those platorms that do not support hard-link, falling back to copy would never be a wrong behavior.

[KKould]

the semantics of hard_link and copy_file are not consistent. For example, when the files of hard_link are modified, the contents will change together, but copy_file will not.

[ethe]

It is not a good formal name, maybe we can choose another one, but the semantic would be good to me.