MEDIUM: session: remove the task pointer from the session

MEDIUM: session: remove the task pointer from the session

Now that the previous changes were made, we can add a struct task
pointer to stream_complete() and get rid of it in struct session.

The new relation between connection, session and task are like this :

          orig -- sess <-- context
           |                   |
           v                   |
          conn -- owner ---> task

Some session-specific parts should now move away from stream.