Entities we get from the state store should always be copied before
altering. Ensure that we copy the volume in the top-level unpublish workflow
before handing off to the steps.

The list stub object for volumes in `nomad/structs` did not match the stub
object in `api`. The `api` package also did not include the current
readers/writers fields that are expected by the UI. True up the two objects and
add the previously undocumented fields to the docs.

Nomad user reported problems with CSI volumes associated with failed
allocations, where the Nomad server did not send a controller unpublish RPC.

The controller unpublish is skipped if other non-terminal allocations on the
same node claim the volume. The check has a bug where the allocation belonging
to the claim being freed was included in the check incorrectly. During a normal
allocation stop for job stop or a new version of the job, the allocation is
terminal. But allocations that fail are not yet marked terminal at the point in
time when the client sends the unpublish RPC to the server.

For CSI plugins that support controller attach/detach, this means that the
controller will not be able to detach the volume from the allocation's host and
the replacement claim will fail until a GC is run. This changeset fixes the
conditional so that the claim's own allocation is not included, and makes the
logic easier to read. Include a test case covering this path.