htlcswitch: TODO for obviating type assertions

memoryMailBox uses multiple container/list.List objects to track
messages and packets, which use interface{} to accept objects of any
type. go1.18 added generics to the language, which means we could use a
typed list instead, allowing us to stop using forced type assertions
when reading objects from the list.

I'm not aware of any standard library implementation of a typed list yet,
so let's just add a TODO for now.

htlcswitch/mailbox.go

@@ -112,6 +112,8 @@ type mailBoxConfig struct {
 
 // memoryMailBox is an implementation of the MailBox struct backed by purely
 // in-memory queues.
+//
+// TODO(morehouse): use typed lists instead of list.Lists to avoid type asserts.
 type memoryMailBox struct {
 	started sync.Once
 	stopped sync.Once