On-disk ledger DB tests.

This is a state-machine based test. The commands here are

• Get the current volatile and immutable tip
• Switch to a fork (possibly rolling back 0 blocks, so equivalent to a push)
• Write a snapshot to disk
• Restore the ledger DB from the snapshots on disk
• Model disk corruption (truncate or delete latest snapshot)

The model here is satisfyingly simple: just a map from blocks to their corresponding ledger state modelling the whole block chain since genesis.