rpcperms+lnd: use macaroon service from interceptor chain

We'll refactor the wallet creation and unlock process in a following
commit and want to make it possible to not need a direct reference to
the macaroon service in our main function. Since we store it in the
interceptor chain anyway (if we're using macaroons in the first place),
we might as well use the instance there directly.

rpcperms/interceptor.go

@@ -386,6 +386,15 @@ func (r *InterceptorChain) AddMacaroonService(svc *macaroons.Service) {
 	r.svc = svc
 }
 
+// MacaroonService returns the currently registered macaroon service. This might
+// be nil if none was registered (yet).
+func (r *InterceptorChain) MacaroonService() *macaroons.Service {
+	r.RLock()
+	defer r.RUnlock()
+
+	return r.svc
+}
+
 // AddPermission adds a new macaroon rule for the given method.
 func (r *InterceptorChain) AddPermission(method string, ops []bakery.Op) error {
 	r.Lock()