Previously, getNotificationCount() only looked at local notifications,
and foreign notifications were added in separately by getMessageCount()
and getAlertCount(). This didn't make any sense and resulted in
counter-intuitive things like I4d49b543.
Instead, add a $global flag to getNotificationCount(). If $global=false,
the local count is returned as before, but if $global=true, the
global count (=local+foreign) is returned. If $global is omitted,
the user's cross-wiki notification preference determines which is returned.
Update getLastUnreadNotificationCount() in the same way, since it had
the same issues.
Also add caching for global counts and timestamps, using a global
memc key.
Bug: T133623
Change-Id: If78bfc710acd91a075771b565cc99f4c302a104d
