* Give the `ScrubVolume()` RPC an option to flag found broken volumes as read-only.
Also exposes this option in the shell `volume.scrub` command.
* Remove redundant test in `TestVolumeMarkReadonlyWritableErrorPaths`.
417051bb slightly rearranges the logic for `VolumeMarkReadonly()` and `VolumeMarkWritable()`,
so calling them for invalid volume IDs will actually yield that error, instead of checking
maintnenance mode first.
e5cf2d2a19