CLEANUP: stconn: rename cs_cant_get() to se_need_more_data()

An equivalent applet_need_more_data() was added as well since that function
is mostly used from applet code. It makes it much clearer that the applet
is waiting for data from the stream layer.
diff --git a/src/conn_stream.c b/src/conn_stream.c
index 3e89512..1695541 100644
--- a/src/conn_stream.c
+++ b/src/conn_stream.c
@@ -489,7 +489,7 @@
 		return NULL;
 	cs_attach_applet(cs, appctx);
 	appctx->t->nice = __sc_strm(cs)->task->nice;
-	cs_cant_get(cs);
+	applet_need_more_data(appctx);
 	appctx_wakeup(appctx);
 
 	cs->state = SC_ST_RDY;