Why need a shared log? Remember the CAP theorem. No need for these bottlenecks. If you want to store that A happened after B, just have A store a (hash of) B.
That's a type of logical clock you're describing (without a partial order over all events, just 2 events). Obviously, if you do that with all events, you will have a logical clock. The hash of the previous event is not a good logical clock, as you cannot define higher level operations over the values, such as - is this event 'newer' than this other event.
