WatchedItem: avoid select/writes for anon user in WatchedItem

@Wikia/platform-team 

https://wikia-inc.atlassian.net/browse/PLATFORM-460

This will save us several millions of queries a day

Backporting https://github.com/wikimedia/mediawiki-core/commit/21884f7537d6562a54241469f378229030b5bfa5#diff-64fe2609dd41ca7d16d8dff717036ed9R140

For a page view of a anon user the following select is executed:

```sql
SELECT  wl_notificationtimestamp  FROM `watchlist`  WHERE wl_user = '0'
AND wl_namespace = '0' AND wl_title = 'Hauptseite'  LIMIT 1
```