chainntfs: flesh out initial draft of interface

* So far very simple, only notifications for tx confirmations, and
outpoint spends.
* Our two cases are: waiting for the funding transaction to reach a
depth of N confirmations, and open channels being notified of the
counterpart trying to cheat them by broadcasting an invalidated
commitment tx.

chainntfs/chainntfs.go

@@ -1,5 +1,7 @@
 package chainntnfs
 
+import "github.com/btcsuite/btcd/wire"
+
 // TODO(roasbeef): finish
 //  * multiple backends for interface
 //   * btcd - websockets
@@ -10,4 +12,14 @@ package chainntnfs
 //   * SPV bloomfilter
 //   * other stuff maybe...
 type ChainNotifier interface {
+	RegisterConfirmationsNotification(txid *wire.ShaHash, numConfs uint32, trigger *NotificationTrigger) error
+	RegisterSpendNotification(outpoint *wire.OutPoint, trigger *NotificationTrigger) error
+
+	Start() error
+	Stop() error
+}
+
+type NotificationTrigger struct {
+	TriggerChan chan struct{}
+	Callback    func()
 }