Store the decided value in the consenus round state and remove value from output message

[ancazamfir]

I was working on #144 and would have liked to change the Decision output of the round state machine to not take a value as it is stored in the state.
However it turns out that this breaks MBT and would require changes in the Quint spec/ state as well afaikt.
Also not clear we need the round in the driver output but this can wait for intergration with external components.

cc @josef-widder @romac