multi: add reset closure to kvdb.View

This commit adds a reset() closure to the kvdb.View function which will
be called before each retry (including the first) of the view
transaction. The reset() closure can be used to reset external state
(eg slices or maps) where the view closure puts intermediate results.

channeldb/forwarding_package_test.go

@@ -786,6 +786,8 @@ func loadFwdPkgs(t *testing.T, db kvdb.Backend,
 		var err error
 		fwdPkgs, err = packager.LoadFwdPkgs(tx)
 		return err
+	}, func() {
+		fwdPkgs = nil
 	}); err != nil {
 		t.Fatalf("unable to load fwd pkgs: %v", err)
 	}