Remove PortUser from introspection and use PortData only

[marthtz]

Brief feature description

Inside introspection classes just store a pointer to PublisherPortData and don't use PublisherPortUser anymore.

Detailed information

Right now the introspection classes (port, mempool, process) are storing PublisherPortUser. These could be removed and created on the fly whenever a PortUser is needed. That'll also eliminate the template usage in the introspection classes further simplifying the introspection and its tests.

[elBoberido]

this makes the tests actually harder, since actual ports will be used and not Mocks. I strongly advise not to do this.