POJO enhancements

[om26er]

• Add multiple Session.publish() implementations
• Add back convenience Session.call() aka max-comfort.

@oberstet We can add more convenience methods to Session.call() as well like you can see for Session.publish(), are you fine with more convenience methods ?

Also it is still not clear how the signature of a method that sends a list of POJOs would look like.

[om26er]

This commit: ce64ca6 introduces two new subscribe methods. First takes a method with single parameter as argument and the second takes two parameters as arguments. Need opinion if we should got with that approach.

[om26er]

Added some new APIs for Session.subscribe() and Session.register() <-- POJO variants.

[om26er]

I pushed more APIs though this branch may not really be fully working yet as there are POJO scenarios that still need to be developed.