add batching for measurement backfill.

src/api/workout.js

@@ -184,7 +184,9 @@ module.exports = function workoutRouter(repo) {
     }
 
     workout.listMeasurements().then((list) => {
-      ws.send(JSON.stringify({workoutStatusBackfill: list}));
+      for (let i = 0; i < list.length; i += 240) {
+        ws.send(JSON.stringify({workoutStatusBackfill: list.slice(i, i+240)}));
+      }
     }).catch(err => {
       ws.send(JSON.stringify({error: `could not list measurements: ${err.message || err}`}));
       ws.close();