Support putIfAbsent() semantics for FineGrainedAtomic map

For Hibernate OGM we work with FineGrainedAtomicMap to represent entity tuples. Now we're looking for a way to insert such map into the cache only if it is not present under the given key yet (so we know after the invocation whether the insert has taken place or not).

Cache#putIfAbsent() seems promising for that, but this seems not usable with FineGrainedAtomicMap (As I seem not able to create the map myself in that case).