Merge makes cached reference stale

[ledsoft]

If an entity is cached as part of an object graph of another entity and them updated separately via em.merge, the reference in cache becomes out of sync and is not evicted, causing subsequent load of the referencing object to contain stale values.

For example, consider objectds a and b, where a has a reference to b.

The following sequence demonstrates the issue:

1. Load a. Both a and b are put into second-level cache.
   2 Clear/close the EntityManager.
2. Edit some attributes of b and merge it into persistence context.
3. JOPA will evict b from cache. However, the b instance referenced by a stays in the cache.
4. Load a. It will be fetched from cache and contain b with now stale values.

[ledsoft]

Fixed in 0.16.3.