We currently use the expiration time to represent the timeout of a
transition. Since we intend to stop treating work priority as a
timeline, we can no longer use this trick.
In this commit, I've changed it to store the event time on the update
object instead. Long term, we will store event time on the root as a map
of transition -> event time. I'm only storing it on the update object
as a temporary workaround to unblock the rest of the changes.
Andrew Clark
b04c7fa28c