gvfsjobunmount: Block new requests before calling unmount() on a thread

Block new requests before calling unmount() on a separate thread to
prevent a race where new jobs are received and processed while the
unmount() is being executed.

This is not necessary for try_unmount() because all the job handling is
done on the same thread as the try_unmount() method and requests are
blocked when the try_unmount() method completes.

https://bugzilla.gnome.org/show_bug.cgi?id=710986