The `prune()` stored procedure takes care of only running the actual `DELETE` statements if nothing has been pruned for `this.config.tokenLifetimes.pruneEvery` milliseconds. i.e. if we run it and the last prune ran a few mins ago, it'll not re-run the deletes, however if `prune()` is called and it last ran successfully over `this.config.tokenLifetimes.pruneEvery` ago, it'll run the deletes.

And now I remember why I added the `property` table, since I also store 'pruneLastRan' in there.