Update the scheduling doc with some information about how async
scheduling works. Also add something about the latency.
Async links add 1 quantum of latency so take that into account when
aggregating latencies.
Also a source directly linked to an async node does not add latency
(we evaluate the tee before incrementing the cycle so that it effectively
is executed in the previous cycle and consumed immediately by async
nodes). We can do this because the driver source always provides data
before the async node, and never concurrently.
Add a listener to the link for the node driver change as well because
that can now influence the latency for async nodes.
