Add AtomicMapLookup method to use in Read Only operation without creating new map if none exists.

AtomicMapLookup declared as the only way AtomicMaps are created/retrieved. But getAtomicMap method has side effect and if no map exists in cache this method creates new one. Sometimes we need to use getAtomicMap in read only fashion without this side effect. One solution is to add Method getAtomicMapForRead that if no entry exists in cache returns something like Collections.emptyMap() and if entry exists returns wrapper like Collections.unmodifiableMap()