Builld scoring data from local channels

[TheBlueMatt]

Some LSPs running LDK want to give users scorer data directly from their node. Sadly, even if they're probing, this data will not include the channels the LSP has, which is fine for the LSP but may result in poor performance for the user. We should have some way to do historical tracking of our own channels and just include that into a scorer, maybe via some future "merge scorer" method.