MINOR: mux-h1: Don't rely on the stream anymore in h1_set_srv_conn_mode()

MINOR: mux-h1: Don't rely on the stream anymore in h1_set_srv_conn_mode()

In h1_set_srv_conn_mode(), we need to get the frontend proxy of a server
connection. untill now, we relied on the stream to get it. But it was a bit
dirty. The stream always exists at this stage but to get it, we also need to get
the stream-interface. Since the commit 7c6f8b146 ("MAJOR: connections: Detach
connections from streams."), the connection's owner is always the session, even
for outgoing connections. So now, we rely on the session to get the frontend
proxy in h1_set_srv_conn_mode().

Use the session instead of the stream to get
the frontend on the server connection