MEDIUM: stream: No longer release backend conn-stream on connection retry

The backend conn-stream is no longer released on connection retry. This
means the conn-stream is detached from the underlying connection but not
released. Thus, during connection retries, the stream has always an
allocated conn-stream with no connection. All previous changes were made to
make this possible.

Note that .attach() mux callback function was changed to get the conn-stream
as argument. The muxes are no longer responsible to create the conn-stream
when a server connection is attached to a stream.
diff --git a/src/http_ana.c b/src/http_ana.c
index 6cb248c..848750c 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -1257,7 +1257,7 @@
 	res->to_forward = 0;
 	res->analyse_exp = TICK_ETERNITY;
 	res->total = 0;
-	si_release_endpoint(&s->si[1]);
+	si_reset_endpoint(&s->si[1]);
 
 	b_free(&req->buf);
 	/* Swap the L7 buffer with the channel buffer */