Have checkpointer send stats once each processing loop.

Noted by Fujii Masao

src/backend/postmaster/checkpointer.c

@@ -527,6 +527,11 @@ CheckpointerMain(void)
 			ckpt_active = false;
 		}
 
+		/*
+		 * Send off activity statistics to the stats collector
+		 */
+		pgstat_send_bgwriter();
+
 		/*
 		 * Nap for a while and then loop again. Later patches will replace
 		 * this with a latch loop. Keep it simple now for clarity.