lib/pull: Do local content imports async too

lib/pull: Do local content imports async too

This came up in <https://github.com/ostreedev/ostree/pull/982>; when
we added more direct local importing, we did it synchronously.

This was actually quite a regression when doing local pulls between different
modes; in particular between a bare mode and `archive`, as we were suddenly
doing gzip {de,}compression in the main thread.

Down the line actually...a simpler fix is probably to change things so that the
local path is really only used when we know we can hardlink; everything else
would go though the fetcher codepath but with `file://`.

But this isn't a lot more code, and the speed/interactivity win is large.

Note we're only doing content async with this patch. We could do metadata as
well; we have the object already local. But the metadata code path is messier,
and metadata objects are smaller.

Another area where this comes up is that in e.g. Fedora releng, most operations
talk to a NetApp via NFS. So this has the classic network filesystem problem
that operations that are normally cheap like `stat()` can actually have
nontrivial latency. Doing as much as possible in threads is better there too.

Closes: #1006
Approved by: jlebon