Fix a race where a soft deleted instance might be removed by mistake

When a soft deleted instance is restored there is a tiny risk that it
might be deleted if the reclaim periodic task runs at the same time.
This happens because the restore operation sets deleted_at to None
which makes the reclaim job think it is ok to delete the instance.

This can be resolved by restricting reclaims to only those instances
whose task_state is None.

Resolves bug 1186243.

Change-Id: Ia138e2d504b7482c46900583f019a32c70513245