CLEANUP: mux-h1: avoid using conn->owner in uncertain areas

CLEANUP: mux-h1: avoid using conn->owner in uncertain areas

Some places use conn->owner to retrieve the session. It's valid because
each time it is done, it's on the frontend, though it's not always 100%
obvious and sometimes requires deep code analysis. Let's clarify these
points and even rely on an intermediary variable to make it clearer. One
case where the owner couldn't differ from the session without being NULL
was also eliminated.