message: follow_me: remove_call_every on destruction
  This is for the case where the user forgets to call stop().
diff: @@ -15,6 +15,9 @@ FollowMeImpl::FollowMeImpl(System& system) : PluginImplBase(system)
  FollowMeImpl::~FollowMeImpl()
  {
  + if (_target_location_cookie) {
  + _parent->remove_call_every(_target_location_cookie);
  + }
  _parent->unregister_plugin(this);
  }
  