In some cases I wasn't seeing this output, and I think it's
because we hadn't `drop()`'d the bar. Since it's in scope
it's cleaner to use its `println` method.

Signed-off-by: Colin Walters <walters@verbum.org>

First, this operation was absolutely not asynchronous
and we need to spawn it in a helper thread on general
principle. Doing that is unfortunately VERY hacky
because we need to clone a lot of things and especially
there's an ergonomics hit here since `Deployment` is
incorrectly `!Send`.

But the main motivation is this operation can be slow sometimes,
so let's show progress output so the admin knows we're not blocked.

Closes: containers#842
Signed-off-by: Colin Walters <walters@verbum.org>

Things we do via this mechanism can take some time; let's
log the elapsed time for greater visibility both to the
human output *and* log to the journal.

Signed-off-by: Colin Walters <walters@verbum.org>