Re-claim forwarded HTLCs on startup

Re-claim forwarded HTLCs on startup

Because `ChannelMonitorUpdate`s can complete asynchronously and
out-of-order now, a `commitment_signed` `ChannelMonitorUpdate` from
a downstream channel could complete prior to the preimage
`ChannelMonitorUpdate` on the upstream channel. In that case, we may
not get a `update_fulfill_htlc` replay on startup. Thus, we have to
ensure any payment preimages contained in that downstream update are
re-claimed on startup.

Here we do this during the existing walk of the `ChannelMonitor`
preimages for closed channels.