Getting fileplans runs into caching issue due to security interceptor

[swagnerfaw]

When RM is enabled and there is a basic RM site, this caching issue occurs, which can lead to unnecessary DB calls.
Every time FilePlanServiceImpl Set<NodeRef> getFilePlans(final StoreRef storeRef) gets called, it is intercepted, and the resulting set gets cleared, when the user does not have read permissions.
The issue occurs when the cached value is returned, because it does not return a copy of the set in the cache, but the set itself. So the GLOBAL shared set inside the cache gets emptied every time it's returned from cache.

[swagnerfaw]

See PR #2440