mod+kvdb+channeldb: use btcwallet new DB interface

Depends on btcsuite/btcwallet#757.
Pulls in the updated version of btcwallet and walletdb that have the DB
interface enhanced by their own View() and Update() methods with the
reset callback/closure supported out of the box. That way the global
package-level View() and Update() functions now become pure redirects.

chainntnfs/btcdnotify/btcd_test.go

@@ -42,7 +42,7 @@ func initHintCache(t *testing.T) *chainntnfs.HeightHintCache {
 	testCfg := chainntnfs.CacheConfig{
 		QueryDisable: false,
 	}
-	hintCache, err := chainntnfs.NewHeightHintCache(testCfg, db)
+	hintCache, err := chainntnfs.NewHeightHintCache(testCfg, db.Backend)
 	if err != nil {
 		t.Fatalf("unable to create hint cache: %v", err)
 	}