invoices+channeldb: Fix AMP invoices behaviour.

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.

invoices/interface.go

@@ -56,6 +56,11 @@ type InvoiceDB interface {
 	// passed payment hash. If an invoice matching the passed payment hash
 	// doesn't exist within the database, then the action will fail with a
 	// "not found" error.
+	// The setIDHint is used to signal whether AMP HTLCs should be fetched
+	// for the invoice. If a blank setID is passed no HTLCs will be fetched
+	// in case of an AMP invoice. Nil means all HTLCs for all sub AMP
+	// invoices will be fetched and if a specific setID is supplied only
+	// HTLCs for that setID will be fetched.
 	//
 	// The update is performed inside the same database transaction that
 	// fetches the invoice and is therefore atomic. The fields to update