BUG/MEDIUM: stream: Be sure to never assign a TCP backend to an HTX stream

BUG/MEDIUM: stream: Be sure to never assign a TCP backend to an HTX stream

With a TCP frontend, it is possible to upgrade a connection to HTTP when the
backend is in HTTP mode. Concretly the upgrade install a new mux. So, once it is
done, the downgrade to TCP is no longer possible. So we must take care to never
assign a TCP backend to a stream on this connection. Otherwise, HAProxy crashes
because raw data from the server are handled as structured data on the client
side.

This patch fixes the issue #420. It must be backported to all versions
supporting the HTX.