All the structs defined in the `channeldb/models` package are graph
related. So once we move all the graph CRUD code to the graph package,
it makes sense to have the schema structs there too. So this just moves
the `models` package over to `graph/db/models`.
Add a new subpackage to `lnd/channeldb` to hold some of the types that
are used in the package itself and in other packages that should not
depend on `channeldb`.
