Option to disable LeakSentry, off in release buidls

[pyricau]

Developers who watch custom objects have the RefWatcher in their production code. It's easy enough to separate it out and have a no-op in its place, but the other option is to add LeakSentry as a dependency (it's small enough). That being said, not everybody will want it turned on by default.

Let's add an option to enable / disable leak sentry reference retaining. By default it'll be on in debuggable builds and off otherwise.