Disabled model auto-refresh when processing email batches

- by default, the model will auto-refresh when you save
- in MySQL, which doesn't support RETURNING, this is implemented by
  doing a SELECT after your actual query
- `email_recipients` is a huge table and `processed_at` is not an
  indexed column, so the query times can become realllyyyyyyy big
- given we don't even need the result of the model save, we can just
  disable fetching all the affected records again
- Bookshelf gives us this ability in the form of `autoRefresh: false`

core/server/services/bulk-email/bulk-email-processor.js

@@ -197,7 +197,7 @@ module.exports = {
             // update all email recipients with a processed_at
             await models.EmailRecipient
                 .where({batch_id: emailBatchId})
-                .save({processed_at: moment()}, Object.assign({}, knexOptions, {patch: true}));
+                .save({processed_at: moment()}, Object.assign({}, knexOptions, {autoRefresh: false, patch: true}));
         }
     },