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/stream.c b/src/stream.c
index 4ca48fe..16456e0 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -1013,7 +1013,7 @@
 	}
 
 	/* Now we can schedule the applet. */
-	cs_cant_get(s->scb);
+	applet_need_more_data(appctx);
 	appctx_wakeup(appctx);
 	return ACT_RET_STOP;
 }