CLEANUP: stconn: rename cs_{check,strm,strm_task} to sc_strm_*

These functions return the app-layer associated with an stconn, which
is a check, a stream or a stream's task. They're used a lot to access
channels, flags and for waking up tasks. Let's just name them
appropriately for the stream connector.
diff --git a/src/log.c b/src/log.c
index a767f59..6eade9a 100644
--- a/src/log.c
+++ b/src/log.c
@@ -3560,7 +3560,7 @@
 {
 	static THREAD_LOCAL struct ist metadata[LOG_META_FIELDS];
 	struct stconn *cs = appctx_cs(appctx);
-	struct stream *s = __cs_strm(cs);
+	struct stream *s = __sc_strm(cs);
 	struct proxy *frontend = strm_fe(s);
 	struct listener *l = strm_li(s);
 	struct buffer *buf = get_trash_chunk();