Enrich ChainDB state machine model with ledger snapshots

At the moment:

- the `UpdateLedgerSnapshots` command does not have any effect on the model, and
- we do not validate the correctness of the ledger-state snapshot taking functionality in the state machine tests

This also introduces an asymmetry in the way `UpdateLedgerSnapshots` is dealt with in the system under test (SUT) and in the model.

Thus, to bring consistency back and to test the ledger-state snapshot taking functionality we need to enrich the model with information about the ledger snapshots.

# Suggested next actions

- Represent snapshots in the model
- Change [`runPure`](https://github.com/input-output-hk/ouroboros-network/blob/f0dcd6620dcea1f344e77c9d3fe1ecf40c361d43/ouroboros-consensus-test/test-storage/Test/Ouroboros/Storage/ChainDB/StateMachine.hs#L600) so that `UpdateLedgerSnapshots` updates the model accordingly
- Change the equality checking between model and SUT so that the ledger snapshots (or some abstraction of them, like slot numbers of snapshots) are compared.