MEDIUM: stream: move the frontend's pointer to the session

Just like for the listener, the frontend is session-wide so let's move
it to the session. There are a lot of places which were changed but the
changes are minimal in fact.
diff --git a/src/backend.c b/src/backend.c
index d56d46f..921dad0 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1090,7 +1090,7 @@
 	}
 
 	/* flag for logging source ip/port */
-	if (s->fe->options2 & PR_O2_SRC_ADDR)
+	if (strm_sess(s)->fe->options2 & PR_O2_SRC_ADDR)
 		s->si[1].flags |= SI_FL_SRC_ADDR;
 
 	/* disable lingering */