Tests of properties of the chain DB model

The model for the chain DB (Test.Ouroboros.Storage.ChainDB.Model) contains a quite a bit of info, but that is primarily because it needs to support stateful APIs such as followers (that follow the tip of the chain) and iterators (which stream a chunk of the chain). The main part of the model is it's model of the volatile DB and the immutable DB, which is again satisfyingly simple: the volatile DB is modelled simply as a set of blocks, and the immutable DB is modelled simply as a list of blocks (i.e., a chain).

Nonetheless, the implementation of the operations on that model is subtle. In particular, the chain DB is responsible for chain selection, and so the model must too. So we have a few properties checking some aspects of the model; in particular, we verify that no matter in which order we add blocks to the chain DB, we always pick the most preferred chain.