spec: Store for committed values and commit certificates as support for ValueSync

[cason]

In order to support the crash-recovery failure model (#578), the layer that adopt consensus should be able to remember the current state of the blockchain, namely the array of committed blocks and associated commit certificates.

The specific requirements should be detailed.

Definition of done

• Add requirement to the ValueSync (English) spec. in quint this is modeled via action updateServer(node, chain). Say that the way this is stored in Quint is because we don't really capture the outside of consensus, i.e., the context/application/etc/

[cason]

Besides of better documenting the solution, can we consider it solved?

[josef-widder]

1. In quint this is modeled in the DriverState.chain, which is a list of ValueStoreEntry, which contains value and commit
2. In the implementation that should most likely be stored outside of consensus and updated upon decisions
3. the entry of height h must be persisted (to stable storage) before consensus of height h+1 is started
4. it must be able to be read by ValueSync (it needs this information to send it to peers so that they can catch up)