LPS-32326 Changing pattern in BaseStagedModelDataHandler slightly [5 minutes to review]

Hey Julio,

I've created a new ticket for this change, as Brian told me he would have more confidence merging things if we keep them separately, hopefully I've not gone too far :)

So it's basically 5 minutes to review hopefully, there is a small change in this PR for the base implementing class for the staged model data handlers.

I wrote some advantages and reasons in the ticket description.

Thanks,

Máté

cc: @KocsisDaniel