Make sure the template composer REST calls synchronize persistence on the same backing jcr nodes

currently, the template composer is synchronized within a single jcr session: This is of course trivial

However, two different browser, clicking at the same time on 'edit' for the same site, might result in concurrently wanting to write to the same jcr nodes... Therefore, locking on the backing identifier of the object (Mount, HstSite, SitemapItem, Component etc) should avoid this race condition