auto dispose objects in the connection scope

[akosyakov]

The connection scope allows to create instances existing only within the user session, such instances can access each other as well as frontend services.

They also get garbage collected when the session is gone, but if an instance implements Disposable when it is responsibility of a developer to dispose it when a connection is closed. It's not trivial and error-prone.

It would be nice if all disposable objects get disposed together with a connection if they were not disposed already.

Motivated by #4521 (review)

cc @svenefftinge @benoitf