Maintain the page ID in WikiPage instead of relying on Title

Also updated WikiPage methods to use it instead of calling Title's
methods directly.

This allows to win a database query for most web requests because
Action::factory() calls WikiPage::getActionOverrides() which will
call exists() through getContentHandler() and getContentModel().
This call to exists() trigerred the object's load from the database,
but this only affected the Title object, so WikiPage needed to load
itself from the database too.
Now this call to exists() will load the WikiPage object, which
populates the LinkCache too.

Change-Id: Iea16d6726ddc2356a25011fed9f2a8dbae3b7b42
Merge "Maintain the page ID in WikiPage instead of relying on Title"