Issue-758: Revise and update Kryo serialization mechanism

- update of the Kryo library to the latest version
- use an external library to get common serializers (for immutable collections)
- simplify the encoding process (no more multiple attempts)

Tested on a basic flow: login for a service, ST validation, logout
I'm almost 100% sure it cannot be ```null```. However, I kept the code "as is" regarding the ```null``` values.

I will add a ```NotNull``` annotation as it doesn't make any sense to encode a ```null``` object.