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/log.c b/src/log.c
index dbcffc6..2c8d20b 100644
--- a/src/log.c
+++ b/src/log.c
@@ -1970,7 +1970,7 @@
 	if (likely(s)) {
 		be = s->be;
 		txn = s->txn;
-		be_conn = cs_conn(s->si[1].cs);
+		be_conn = cs_conn(s->csb);
 		status = (txn ? txn->status : 0);
 		s_flags = s->flags;
 		uniq_id = s->uniq_id;