* Implement & document optimization (although legacy kind doesn't do much of it)
* Introduce `optimize_target_tasks` parameter to control whether tasks in the
target set can be optimized (no for try, yes for most other branches)
* Refactor to include resolved taskIds in the optimized task graph
* Include a `label-to-taskid.json` artifact.
* Introduce {'task-reference': '... <dependency-name> ...'} for referring to
parent tasks' taskId.
MozReview-Commit-ID: LWvlWNz49U5
