Progress reporting: include origins of running actions

For a user, an important information is to know which actions
are currently running and, more importantly, the target that
caused them. To do so, we need a bit of infrastructure.
- We have to keep track of begin and end of running actions,
  as well as the order in which they were started. That has
  to happen efficiently and in a thread-safe way.
- We have to compute and keep the origin map for actions,
  even if we don't serialize the action graph.

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