Yannick Welsch 0f65390c29 Do not mutate engine during planning step (#39571)
This cleans up the Engine implementation by separating the sequence number generation from the
planning step in the engine, to avoid for the planning step to have any side effects. This makes it
easier to see that every sequence number is properly accounted for.
2019-03-04 10:11:39 +01:00
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