`MonitorUpdatingPersister` apparently reads only no-update monitors when archiving

[TheBlueMatt]

...it should read with updates!

[yellowred]

When a channel is being archived using MonitorUpdatingPersister it only reads channel monitor and not channel monitor updates. This works today because of the external assumptions, ie full channel monitor is persisted on every block and after the channel was closed, but there is no guarantee that some change in the future will not break those assumptions and consequentially MonitorUpdatingPersister.

[tnull]

Probably might be worth to fix this in conjunction with #3121 and #3122, as #3121 would solve the issue as we'd make sure to persist the full monitor when first initiating the archival.