splitstore shed utils

[vyzo]

Adds a lotus-shed splitstore command with some utilities:

• rollback command, which rolls back the splitstore.
• check which performs a health check.
• info which prints some basic information.

Closes #6737

[magik6k]

Would be nice to also have a light info command which would get you blockstore sizes, and maybe some state (e.g. epochs in hotstore + maybe some things which could be useful for debugging user nodes)

[vyzo]

sure!

[raulk]

Looks sane so far.

[jacobheun]

@vyzo can you ensure that https://github.com/filecoin-project/lotus/blob/master/blockstore/splitstore/README.md is updated to include a section on how to rollback when this is ready?

[vyzo]

ah yes, thanks for reminding me -- will do.

[vyzo]

rebased on master.

[vyzo]

rebased on master.

[magik6k]

Looks good, just one nit about perms