db vacate & purge

[mccanne]

add zed lake vacate command stubbed out and described in cmd/zed/lake/vacate/command.go

The Zed lake design doc also proposes a zed lake purge command that's subtly different from vacate, and that one's not yet implemented either. So as part of the design we'll surely figure out if both have to exist and implement accordingly.

[philrz]

Note to self: When this gets implemented, update the Zed lake design doc to remove the reference to this being an open isssue.

[philrz]

Closing as a duplicate of #4113

[philrz]

I mistakenly thought vacuum did the same thing envisioned by vacate so had closed this one in error. Reopening.