MEDIUM: connection: add a destroy callback

MEDIUM: connection: add a destroy callback

This callback will be used to release upper layers when a mux is in
use. Given that the mux can be asynchronously deleted, we need a way
to release the extra information such as the session.

This callback will be called directly by the mux upon releasing
everything and before the connection itself is released, so that
the callee can find its information inside the connection if needed.

The way it currently works is not perfect, and most likely this should
instead become a mux release callback, but for now we have no easy way
to add mux-specific stuff, and since there's one mux per connection,
it works fine this way.