CLEANUP: applet: remove the unneeded appctx->owner

This one is the pointer to the conn_stream which is always in the
endpoint that is always present in the appctx, thus it's not needed.
This patch removes it and replaces it with appctx_cs() instead. A
few occurences that were using __cs_strm(appctx->owner) were moved
directly to appctx_strm() which does the equivalent.
diff --git a/src/mworker.c b/src/mworker.c
index 1ee93fb..24fde3c 100644
--- a/src/mworker.c
+++ b/src/mworker.c
@@ -509,7 +509,7 @@
 /*  Displays workers and processes  */
 static int cli_io_handler_show_proc(struct appctx *appctx)
 {
-	struct conn_stream *cs = appctx->owner;
+	struct conn_stream *cs = appctx_cs(appctx);
 	struct mworker_proc *child;
 	int old = 0;
 	int up = now.tv_sec - proc_self->timestamp;