R1:2c5b1dc20a6b

Provide "--output" flags for "bin/storage renamespace"

Summary:
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…