chainntnfs+chainreg: add interface MempoolWatcher

This commit adds the interface `MempoolWatcher` and uses it in the chain
registry.

chainreg/chainregistry.go

@@ -182,6 +182,10 @@ type PartialChainControl struct {
 	// interested in.
 	ChainNotifier chainntnfs.ChainNotifier
 
+	// MempoolNotifier is used to watch for spending events happened in
+	// mempool.
+	MempoolNotifier chainntnfs.MempoolWatcher
+
 	// ChainView is used in the router for maintaining an up-to-date graph.
 	ChainView chainview.FilteredChainView
 
@@ -433,10 +437,14 @@ func NewPartialChainControl(cfg *Config) (*PartialChainControl, func(), error) {
 				"bitcoind: %v", err)
 		}
 
-		cc.ChainNotifier = bitcoindnotify.New(
+		chainNotifier := bitcoindnotify.New(
 			bitcoindConn, cfg.ActiveNetParams.Params, hintCache,
 			hintCache, cfg.BlockCache,
 		)
+
+		cc.ChainNotifier = chainNotifier
+		cc.MempoolNotifier = chainNotifier
+
 		cc.ChainView = chainview.NewBitcoindFilteredChainView(
 			bitcoindConn, cfg.BlockCache,
 		)
@@ -655,7 +663,8 @@ func NewPartialChainControl(cfg *Config) (*PartialChainControl, func(), error) {
 			DisableConnectOnNew:  true,
 			DisableAutoReconnect: false,
 		}
-		cc.ChainNotifier, err = btcdnotify.New(
+
+		chainNotifier, err := btcdnotify.New(
 			rpcConfig, cfg.ActiveNetParams.Params, hintCache,
 			hintCache, cfg.BlockCache,
 		)
@@ -663,6 +672,9 @@ func NewPartialChainControl(cfg *Config) (*PartialChainControl, func(), error) {
 			return nil, nil, err
 		}
 
+		cc.ChainNotifier = chainNotifier
+		cc.MempoolNotifier = chainNotifier
+
 		// Finally, we'll create an instance of the default chain view
 		// to be used within the routing layer.
 		cc.ChainView, err = chainview.NewBtcdFilteredChainView(