Support lazy initialization using atomic variables

In the new concurrent package there is already the LazyInitializer class implementing the double-check idiom for lazy initialization of instance fields. Another way to perform initialization lazily would be the usage of atomic variables. This solution would not need synchronization at all. If there is low contention (not many threads in parallel), it is more efficient than synchronized access. Its drawback is that the initialization may be done multiple times if multiple threads access the initializer almost at the same time (however, it can be guaranteed that always the same object created by the initializer is returned).

This is a proposal to add an AtomicInitializer class as an alternative to LazyInitializer that operates on AtomicReference variables.

In order to make the initializers smoothly exchangable, it would make sense for them to share a common interface. This interface could also be implemented by the BackgroundInitializer class.