o RooAbsOptDataStore

   - Make function hasFilledCache() virtual to allow for non-broken
     actual implementation in derived classes

   - Add new virtual method forceCacheUpdate() to request wholesale updating
     of all caches whenever recalculateCache() is called the next time

 o RooCompositeDataStore

   - Implement forwarding version of hasFilledCache()

   - Implement forwarding version of forceCacheUpdate()

 o RooVectorDataStore

   - Implement working version of hasFilledCache()

   - Add method forceCacheUpdate() that will trigger a wholesale
     update of all cached functions, including caches for functions
     that only depend on constant parameters

 o RooAbsOptTestStatistics

   - Chanche implementation of ValueChange operation in constOptimizeTestStatistic():
     instead of cycling (deactivate + activate), simply request a forced wholesale cache
     update for the next evaluation of the likelihood using RooVectorDataStore::forceCacheUpdate()