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/connection.c b/src/connection.c
index e4d26dd..53ac9a6 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -1188,11 +1188,11 @@
 		 * send a LOCAL line (eg: for use with health checks).
 		 */
 
-		if (cs && cs_strm(cs)) {
+		if (cs && sc_strm(cs)) {
 			ret = make_proxy_line(trash.area, trash.size,
 					      objt_server(conn->target),
 					      cs_conn(cs_opposite(cs)),
-					      __cs_strm(cs));
+					      __sc_strm(cs));
 		}
 		else {
 			/* The target server expects a LOCAL line to be sent first. Retrieving