FineGrainedAtomicMap doesn't currently work with write skew check enabled.

I was able to make it work by adding a special case for DeltaAwareCacheEntry in WriteSkewHelper, however the map has a single version, so the write skew check fails if any of the sub-keys were modified in parallel. With pessimistic locking, fine-grained maps allow the user to modify different sub-keys concurrently, we should allow the same with optimistic locking.