We erase peer data after the last channel close transaction for that
peer is 100 blocks deep. We were failing to finish the migration because
the peer_id lookup on these was failing.
Now we ignore any channel with a null peer_id.
Fixes#3768
We use a database snapshot with 3 channels -- two of which have HTLCs
dangling and one is an initial open channel tx in the 'old' tx hex
format in last_tx and confirm that they are successfully updated to PSBT
format on start.
