utxonursery: properly set height hint when recovering commit sweep
utxonursery: properly set height hint when recovering commit sweep

This commit alters the restart logic in the nursery
to load channel close summaries from disk when
restarting the process of sweeping commit txns.
The channel close summary contains a closing height,
which is used to set the lower bound of the height hint
when resubscribing to spend notifications.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd