Closes #5993

Objects implementing RDestroyable (such as RMapCache, RLocalCachedMap, RSetCache, RDelayedQueue, etc.) require explicit destruction to avoid leaking timer tasks, evictions, or listener subscriptions.

This PR makes RDestroyable extends AutoCloseable with a default method:

@Override
default void close() {
    destroy();
}

• Enables standard Java try-with-resources across all RDestroyable structures without declaring checked exceptions.
• 100% backwards-compatible with existing code invoking .destroy().
• Added RDestroyableTest unit test.