private void addWatcher() {
            if (useJrtWatcher) {
                request.getRequest().target().addWatcher(this);
            }
        }
Removes this delayed response if target is invalid.

@param target a Target that has become invalid (i.e, client has closed connection)
@see DelayedConfigResponses
configserver/src/main/java/com/yahoo/vespa/config/server/rpc/DelayedConfigResponses.java