Immutable DB tests.

The immutable DB bundles a (configurable) number of blocks into "chunk files". By design, chunk files are literally just the raw blocks, one after the other, so that we can efficiently support binary streaming of blocks.

Every chunk file is accompanied by two indices: a _primary_ index that for each slot in the chunk file provides an offset into a _secondary_ index, which stores some derived information about the blocks in the chunk file for improved performance. Both the primary and the secondary index can be reconstructed from the chunk file itself.

The tests for the immutable DB consist of a handful of unit tests, a set of property tests of the primary index, and then the main event, model based checking.