Make sure a ReferenceStat is set to values of the correct type (#6430)

Make sure a ReferenceStat is set to values of the correct type (#6430)

This PR fixes a very subtle issue with setting the values a ReferenceStat refers to.
ReferenceStat::set() would take a variable by const& and then store the pointer to it. When giving it a different, but implicitly convertible, type, the pointer would assume the wrong type and consequently read incorrect values from it.
This PR makes set() a template function that explicitly checks that the given type is the correct one.
As we can only export int64_t to the API, this forces users of ReferenceStat to use int64_t stats.