MEDIUM: muxes: always set conn->owner to the session that owns the connection

MEDIUM: muxes: always set conn->owner to the session that owns the connection

When an idle connection is private or considered private, session_add_conn()
is called to add it to the list of connections owned by the session. But
in case of allocation failure, the session is not set, which results in
a long list of possible situations that are all corner cases which are
difficult to test (and debug).

This commit relies on the fact that it is already permitted to have
conn->owner pointing to a session even if the connection couldn't be
added to the session's list, as this was already the case in
conn_backend_get() when dealing with HOL_RISK. Also as seen in commit
3aab17bd566 added in 2.4, it is already possible to have conn->owner
set with the connection not being in a list, and only the list element
is checked for this.

This commit modifies session_add_conn() to always set conn->onwer, even
if the list element couldn't be allocated. This way it's possible to
always refer to conn->owner to find the session owning a private conn
even in case of failure to allocate an entry. This requires to change
the checks on conn->owner to a check of the list element to see if the
connection belongs to a session, the pre-assignment of sess to
conn->owner in conn_backend_get() is no longer needed, same for the
pre-assignment in http_wait_for_response(), and that's all.

The H1 mux remained unchanged because since it cannot multiplex, in
case it fails to allocate a pconn, it instantly kills the connection.