block_writer: update for breaking change in satori/go.uuid API

[pib]

As mentioned at satori/go.uuid#66, the satori/go.uuid API changed, so the call to uuid.NewV4 needs to be wrapped in uuid.Must.

[cockroach-teamcity]

This change is 

[petermattis]

@pib Thanks for the fix. This binary isn't under active use any more. We should probably just delete it, but might as well merge this PR until that is done.

[pib]

@petermattis I came upon it because it is mentioned in the docs at https://www.cockroachlabs.com/docs/stable/demo-automatic-rebalancing.html, so y'all may want to update that page as well.

[petermattis]

@pib Thanks for the docs pointer. We'll get that fixed.

[bdarnell]

Also, as discussed in cockroachdb/cockroach#26332, the API change also introduced a severe bug (at least on macos) that has not been fixed (the maintainer has been unresponsive), so any users of go.uuid should pin to an older version or switch to a different package.