Use data from master when recalculating statistics after a translation update
Use TranslateUtils::getSafeReadDB() (which uses master during update requests)
in MessageCollection::load* methods.
PageTranslationHooks::sourceExport() will now probably load from slave but
I don't think this is a critical path which needs the latest data from master
so this is probably okay.
Bug: T49864
Change-Id: I7070af156bd19044982f0baec7902895806f4807