chainntnfs: add Updates channel field to ConfirmationEvent

In this commit, we add a new Updates channel to our ConfirmationEvent
struct. This channel will be used to deliver updates to a subscriber of
a confirmation notification. Updates will be delivered at every
incremental height of the chain with the number of confirmations
remaining for the transaction to be considered confirmed by the
subscriber.

chainntnfs/interface.go

@@ -85,8 +85,10 @@ type TxConfirmation struct {
 
 // ConfirmationEvent encapsulates a confirmation notification. With this struct,
 // callers can be notified of: the instance the target txid reaches the targeted
-// number of confirmations, and also in the event that the original txid becomes
-// disconnected from the blockchain as a result of a re-org.
+// number of confirmations, how many confirmations are left for the target txid
+// to be fully confirmed at every new block height, and also in the event that
+// the original txid becomes disconnected from the blockchain as a result of a
+// re-org.
 //
 // Once the txid reaches the specified number of confirmations, the 'Confirmed'
 // channel will be sent upon fulfilling the notification.
@@ -100,6 +102,11 @@ type ConfirmationEvent struct {
 	// details of the channel's confirmation.
 	Confirmed chan *TxConfirmation // MUST be buffered.
 
+	// Updates is a channel that will sent upon, at every incremental
+	// confirmation, how many confirmations are left to declare the
+	// transaction as fully confirmed.
+	Updates chan uint32 // MUST be buffered.
+
 	// TODO(roasbeef): all goroutines on ln channel updates should also
 	// have a struct chan that's closed if funding gets re-org out. Need
 	// to sync, to request another confirmation event ntfn, then re-open