Improve Hazelcast serialization

[vpavic]

HazelcastSessionRepository currently relies on standard Java serialization, which isn't the ideal option performance wise. We should look into leveraging Hazelcast's serialization mechanism like DataSerializable which should yield a nice performance improvement.

[oxc]

In order to fix the issue mentioned in #1319 (client-server topology with custom-class session entries), the solution must ensure that no session entries are deserialized on the server-side.
Switching to another form of serialization will probably not provide this without changing what is stored inside the session map, especially since the EntryProcessor holding the session delta must itself be java Serializable.