Ref T6996. Depends on D16407. This does the same stuff as D16407, but for bin/storage renamespace. In particular:
- Support writing directly to a file (so we can get good errors on failure).
- Support in-process compression.
Also add support for reading out of a storage dump subprocess, so we don't have to do a dump-to-disk + renamespace + compress dance and can just stream out of MySQL directly to a compressed file on disk.
This is used in the second stage of instance exports (see T7148).
It would be nice to share more code with bin/storage dump, and possibly to just make this a flag for it, although we still do need to do the file-based version when importing (vs exporting). I figured that was better left for another time.