Make PackageTemplate::height_timer non-optional

Make PackageTemplate::height_timer non-optional

Now that we leverage a package's `height_timer` even for untractable
packages, there's no need to have it be an `Option` anymore. We aim to
not break compatibility by keeping the deserialization of such as an
`option`, and use the package's `height_original` when not present. This
allows us to retry packages from older `ChannelMonitor` versions that
have had a failed initial package broadcast.