REORG/MEDIUM: stream: rename stream flags from SN_* to SF_*

This is in order to keep things consistent.
diff --git a/src/peers.c b/src/peers.c
index a2f29d2..6df3ce7 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1125,7 +1125,7 @@
 	LIST_INIT(&s->back_refs);
 	LIST_INIT(&s->buffer_wait);
 
-	s->flags = SN_ASSIGNED|SN_ADDR_SET;
+	s->flags = SF_ASSIGNED|SF_ADDR_SET;
 
 	/* if this session comes from a known monitoring system, we want to ignore
 	 * it as soon as possible, which means closing it immediately for TCP.