In-memory ledger DB tests.

The in-memory component of the ledger DB is a bit tricky: it stores only a few snapshots of the ledger state, in order to reduce memory footprint, but must nonetheless be able to construct any ledger state (within k blocks from the chain tip) efficiently. The properties we are verify here are various invariants of this data type, things such as

• Rolling back and then reapplying the same blocks is an identity operation (provided the rollback is not too far)
• The shape of the datatype (where we store snapshots and how many we store) always matches the policy set by the user, and is invariant under any of the operations (add a block, switch to a fork, etc.)
• The maximum rollback supported is always k (unless we are near genesis)
• etc.