Transactional caches are being bypassed

As part of CONFDEV-24710, a change was made on the 5.5-stable branch that fundamentally changed the way that some of the application code interacts with the TransactionalCacheFactory. This was done under the banner of cluster cache optimisation, but it in effect it completely bypasses the transactional semantics of the caches in question.

The most noticeable effect of this is a recurrence of the "user already exists" and "directory removed during iteration" errors that used to plague the builds before they were fixed by the introduction of transactional caches.

While this change did go into stable, at this point we do not believe that it is likely to have a noticeable impact on production systems. If it does, it'll most likely have similar impacts to the test failure messages mentioned above, where caches related to users or user directories go stale.

However, we should be alive to the possibility that customers will report issues along these lines (cc shaffenden).

For now, we'll get this fixed ASAP on stable and get it merged to master toot sweet. This should go into Thunder26 and 5.5.3.