* Process deletions conservatively in parallel
This commit allows the engine to conservatively delete resources in
parallel when it is sure that it is legal to do so. In the absence of a
true data-flow oriented step scheduler, this approach provides a
significant improvement over the existing serial deletion mechanism.
Instead of processing deletes serially, this commit will partition the
set of condemned resources into sets of resources that are known to be
legally deletable in parallel. The step executor will then execute those
independent lists of steps one-by-one until all steps are complete.
* CR: Make ResourceSet a normal map
* Only use the dependency graph if we can trust it
* Reverse polarity of pendingDeletesAreReplaces
* CR: un-export a few types
* CR: simplify control flow in step generator when scheduling
* CR: parents are dependencies, fix loop index
* CR: Remove ParentOf, add new test for parent dependencies
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