WatchedItem: avoid select/writes for anon user in WatchedItem

This will save us several milions 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:
SELECT  wl_notificationtimestamp  FROM `watchlist`  WHERE wl_user = '0'
AND wl_namespace = '0' AND wl_title = 'Hauptseite'  LIMIT 1
Do not load special pages in WatchedItem::load

Backporting https://github.com/wikimedia/mediawiki-core/commit/607449cb84e20c5b1035f4b6867b3d1ac6b26567#diff-64fe2609dd41ca7d16d8dff717036ed9R32

WatchedItem::load was looking for a special page in the database:

SELECT  wl_notificationtimestamp
FROM `watchlist`
WHERE wl_user = '1'
AND wl_namespace = '-1'
AND wl_title = 'Log'
LIMIT 1

But it is known that such a page never will stay in the database,
so added a check to avoid the select.
Merge pull request #4770 from Wikia/watcheditem-ignore-anons

WatchedItem: avoid select/writes for anon user in WatchedItem