servers when it is changed/removed. Removes RemoveByJson as we only actually require RefreshByJson since we can put any sort of parameters
in a custom json string including whether it is a remove operation (if required)
cleared when media was deleted. Created base classes for cache refreshers, we now have a new event - CacheUpdated
which can now be used by code to execute on each individual server when any cache refresher is updated. Listening to events
normally only fire on the individual server so if people are wanting to refresh their own cache there was previously no way
to do that.
