MINOR: stream: Add pointer to front/back conn-streams into stream struct

frontend and backend conn-streams are now directly accesible from the
stream. This way, and with some other changes, it will be possible to remove
the stream-interfaces from the stream structure.
diff --git a/src/cache.c b/src/cache.c
index ac4c682..b4918e7 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -2649,7 +2649,7 @@
 		return 0;
 
 	/* Get appctx from the stream_interface. */
-	appctx = cs_appctx(smp->strm->si[1].cs);
+	appctx = cs_appctx(smp->strm->csb);
 	if (appctx && appctx->rule) {
 		cconf = appctx->rule->arg.act.p[0];
 		if (cconf) {