MEDIUM: session: always reset the conn->owner on backend when installing mux

MEDIUM: session: always reset the conn->owner on backend when installing mux

When installing a mux on the backend, unless we have a good reason for
keeping the session set in conn->owner, we must reset it. Having the
session there just hides potential bugs and prevents certain tests from
being properly done.

Now it is much clearer: conn->owner remains set to the session on
frontend connections, is set to the session when the connection is
private or assimilated private and belongs to the session list, or
is NULL.