Hazelcast implementation of `ReactiveSessionRepository`

[vpavic]

We should provide a Hazelcast based implementation of ReactiveSessionRepository.

[vpavic]

While Hazelcast currently has reactive support, it is based on its own ICompletableFuture which extends from Future and not CompletableFuture since Hazelcast 3.x maintains compatibility with Java 6.

The plan is to replace ICompletableFuture with CompletableFuture in Hazelcast 4.0 when Java 8 becomes baseline - see hazelcast/hazelcast#3622.

Perhaps we should hold off our efforts of providing Hazelcast backed ReactorSessionRepository until Hazelcast 4.0 becomes available (ATM there's no target date for the milestone). WDYT @rwinch?

[hantsy]

The hazelcast itself have Spring integration in the hazelcast core, I am not sure if it works when implementing a custom MapReactiveSessionRepository with a hazelcast managed Map as the constructor arguments ?