multi: make macaroon DB remote compatible

The macaroon root keys should also be stored to the remote database if a
replicated backend such as etcd is used.
This commit refactors the macaroons service and wallet unlocker to
accept a kvdb backend directly instead of creating the bolt instance
automatically.

macaroons/store_test.go

@@ -54,6 +54,7 @@ func openTestStore(t *testing.T, tempDir string) (func(),
 
 	cleanup := func() {
 		_ = store.Close()
+		_ = db.Close()
 	}
 
 	return cleanup, store
@@ -108,6 +109,7 @@ func TestStore(t *testing.T) {
 	require.Equal(t, macaroons.ErrAlreadyUnlocked, err)
 
 	_ = store.Close()
+	_ = store.Backend.Close()
 
 	// Between here and the re-opening of the store, it's possible to get
 	// a double-close, but that's not such a big deal since the tests will
@@ -206,6 +208,8 @@ func TestStoreChangePassword(t *testing.T) {
 	// after closing.
 	err = store.Close()
 	require.NoError(t, err)
+	err = store.Backend.Close()
+	require.NoError(t, err)
 
 	err = store.CreateUnlock(&newPw)
 	require.Error(t, err)