`MockChainSt` vs `EmulatedLedgerState`

[mmontin]

For now we use our own internal state to keep track of the required info during a mockchain run. Since we rely on cardano-node-emulator which uses EmulatedLedgerState we could rely on it directly instead of transferring info from one to the other. We need to investigate if that's possible and relevant, and also ensure that cardano-node-emulator will be maintained if we do so.