Fix set of flaky tests in the HST Page Composer

A set of tests in the page-composer, most notably CRUDPageAndModifyContainerTest fail frequently. This is due to JCR async behavior wrt when other sessions see each others changes. The only simple 'fix' for this is to make sure after a session.save some waiting is done. In 75% of the places we already do this, but in quite some places we do not do this properly