MINOR: session: strengthen connection attach to session

MINOR: session: strengthen connection attach to session

This commit is the first one of a serie to refactor insertion of backend
private connection into the session list.

session_add_conn() is used to attach a connection into a session list.
Previously, this function would report an error if the connection
specified was already attached to another session. However, this case
currently never happens and thus can be considered as buggy.

Remove this check and replace it with a BUG_ON(). This allows to ensure
that session insertion remains consistent. The same check is also
transformed in session_check_idle_conn().