Tests for the garbage collection schedule.

The real implementation of course is stateful, running some code at various intervals. We have a model of the implementation along with some QuickCheck properties checking various invariants of the model such as

• The length of the queue is bounded
• The overlap between the volatile DB and immutable DB (due to blocks that could have been moved but haven't yet) is bounded.

We then test that the real implementation behaves exactly as the model predicts.