* Execute chains of steps in parallel
Fixespulumi/pulumi#1624. Since register resource steps are known to be
ready to execute the moment the engine sees them, we can effectively
parallelize all incoming step chains. This commit adds the machinery
necessary to do so - namely a step executor and a plan executor.
* Remove dead code
* CR: use atomic.Value to be explicit about what values are atomically loaded and stored
* CR: Initialize atomics to 'false'
* Add locks around data structures in event callbacks
* CR: Add DegreeOfParallelism method on Options and add comment on select in Execute
* CR: Use context.Context for cancellation instead of cancel.Source
* CR: improve cancellation
* Rebase against master: execute read steps in parallel
* Please gometalinter
* CR: Inline a few methods in stepExecutor
* CR: Feedback and bug fixes
1. Simplify step_executor.go by 'bubbling' up errors as far as possible
and reporting diagnostics and cancellation in one place
2. Fix a bug where the CLI claimed that a plan was cancelled even if it
wasn't (it just has an error)
* Comments
* CR: Add comment around problematic select, move workers.Add outside of goroutine, return instead of break
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