[MCR] Move getSecondaryDataUpdates to the page level
Replaces Content::getSecondaryDataUpdates with
WikiPage::getSecondaryDataUpdates so that aggregation of
data updates from multiple page slots can be handled without
the caller having to care about it.
Also adds a WikiPage::updateParserCache method for convenience.
This is a temporary measure until DerivedPageDataUpdater
(or its replacement) can be exposed directly, at which point
the WikiPage methods will be deprecated.
Also fixes a parameter handling bug in DerivedPageDataUpdater.