src: save the performance milestone time origin in the AliasedArray
Previously we cache the time origin for the milestones in the user land, and refresh it at pre-execution. As result the time origin gets serialized into the snapshot and is therefore not deterministic. Now we store it in the milestone array as an internal value and reset the milestones at serialization time instead of deserialization time. This improves the determinism of the snapshot.
Drive-by: remove the unused MarkMilestone() binding.