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/pool.c b/src/pool.c
index 8352531..2b4754e 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -1011,7 +1011,7 @@
  */
 static int cli_io_handler_dump_pools(struct appctx *appctx)
 {
-	struct conn_stream *cs = appctx->owner;
+	struct conn_stream *cs = appctx_cs(appctx);
 
 	dump_pools_to_trash();
 	if (ci_putchk(cs_ic(cs), &trash) == -1) {