CLEANUP: conn_stream: rename the cs_endpoint's context to "conn"

This one is exclusively used by the connection, regardless its generic
name "ctx" is rather confusing. Let's make it a struct connection* and
call it "conn". This way there's no doubt about what it is and there's
no way it will be used by accident by being taken for something else.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 9ed2727..cd57d98 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -1613,7 +1613,7 @@
 	if (!h2s->endp)
 		goto out_close;
 	h2s->endp->target = h2s;
-	h2s->endp->ctx = h2c->conn;
+	h2s->endp->conn   = h2c->conn;
 	h2s->endp->flags |= (CS_EP_T_MUX|CS_EP_ORPHAN|CS_EP_NOT_FIRST);
 
 	/* FIXME wrong analogy between ext-connect and websocket, this need to