Ability to assert on stat lifetime issues in debug build

There have been some issues recently with stat references (`Counter&` et al) outliving their owning `Scope`. This can lead to use-after-free.

One way to help detect this would be for `Counter&` and friends to be replaced with a class `CounterRef`. This class, in a release build, would only contain a `Counter&`, to avoid a huge increase in memory use. But in debug builds. This object would implement the `Counter` interface, passing all calls through to the stored `Counter&`.

In a debug build, this object would also hold a reference to the `Scope` that created it, and call a new method on the `Scope` to inform that it is using a stat from that `Scope`. In `Scope`'s destructor, it would ASSERT if it is still in use by anything.

Hopefully, the macros that create most stats could be modified to work for this, instead of needing to touch all code creates stats.