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/http_client.c b/src/http_client.c
index bff7057..1efb484 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -195,7 +195,7 @@
 static int hc_cli_io_handler(struct appctx *appctx)
 {
 	struct hcli_svc_ctx *ctx = appctx->svcctx;
-	struct conn_stream *cs = appctx->owner;
+	struct conn_stream *cs = appctx_cs(appctx);
 	struct buffer *trash = alloc_trash_chunk();
 	struct httpclient *hc = ctx->hc;
 	struct http_hdr *hdrs, *hdr;
@@ -739,7 +739,7 @@
 static void httpclient_applet_io_handler(struct appctx *appctx)
 {
 	struct httpclient *hc = appctx->svcctx;
-	struct conn_stream *cs = appctx->owner;
+	struct conn_stream *cs = appctx_cs(appctx);
 	struct stream *s = __cs_strm(cs);
 	struct channel *req = &s->req;
 	struct channel *res = &s->res;