Chain DB tests.

The chain DB is the most complicated part of the storage layer, and this is visible in the tests also. We have separate tests for:

• The GC schedule: the chain database occassionally garbage collects blocks from the volatile DB that it is sure it won't need anymore (because adopting them would result in switching to a fork that is too distant from our own chain).
• Iterators. Iterators are cursors that allow to read a sequence of blocks from the DB. The iterators provided by the chain DB can span both the immutable DB and the volatile DB, some of those blocks might be moved from the volatile DB to the immutable DB during the iterator's life-time, and some of those blocks might be garbage collected from the volatile DB. This makes the iterator code quite complex, having to deal with a lot of edge cases. Their main tests are part of the model based test, but we also test some specific properties of the iterators themselves.
• The model of the chain DB itself is reasonably complex also, and so we have some properties that verify that that the model behaves the way we think it should.
• The main DB itself.