R1:924b135d310b

Add a `storage renamespace` for mangling SQL dumpfiles into a new namespace

Summary:
Ref T7149. When users give us dumpfiles for import, they will almost inevitably use the `phabricator` namespace. They need to be renamed to use an instance namespace.

We can do this either by:

- importing the data first, then renaming; or
- renaming first, then importing.

This implements the second one, basically `storage renamespace --in dump.sql --from phabricator --to instancename > instance.sql`.

Renaming…