We now cancel all HTLCs of an AMP invoice as soon as it expires.
Otherwise because we mark the invoice as cancelled we would not
allow accepted HTLCs to be resolved via the invoiceEventLoop.
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`.
With the introducation of the `InvoiceUpdater` interface we are now
able to move the non-kv parts of `UpdateInvoice` completely under
the invoices package. This is a preprequisite for being able to use
the same code-base for the sql InvoiceDB implementation of
UpdateInvoice.
