Clear SiteStats process cache after DB update

Clear SiteStats process cache after DB update

This fixes a phpunit test error, wherein {{NUMBEROFFILES}} would give
the number of files in the host wiki, not in the temporary database,
when Scribunto was installed, due to a Scribunto phpunit data provider
calling SiteStats::pages().

Change-Id: Ic0d021a72addaa2a13a6b94fd34dccc423de3a8f