oauthlib: add method to set a token as expired

This can be used by backends to trigger a refresh of an access token if
they detect an invalid token.

lib/oauthutil/renew.go

@@ -67,3 +67,8 @@ func (r *Renew) Stop() {
 func (r *Renew) Invalidate() {
 	r.ts.Invalidate()
 }
+
+// Expire expires the token source
+func (r *Renew) Expire() error {
+	return r.ts.Expire()
+}