Speed improvement: Delete presences older than one week, and add index to presences serverId.

Thanks to xet7 !

Related #3345

models/presences.js

@@ -0,0 +1,12 @@
+if (Meteor.isServer) {
+  Meteor.startup(() => {
+    // Date of 7 days ago
+    let lastWeek = new Date();
+    lastWeek.setDate(lastWeek.getDate() - 7);
+
+    presences.remove({ ttl: { $lte: lastWeek } });
+
+    // Create index for serverId that is queried often
+    presences._collection._ensureIndex({ serverId: -1 });
+  });
+}