Dependency only tasks should run with maximum allowed priority of their dependents #625

Tasks that get pulled into the graph only as a dependency should adjust their priority to match the priority of the task(s) that depend on it.

For example, if a test task runs with the lowest priority, the build that it depends on should also be lowest priority. Conversely, if we're in the middle of a chemspill and need to schedule some release tasks with highest priority, the builds that they depend on should also run with highest priority.

In Gecko, we already have the ability to set the priority of an entire graph, so the chemspill case isn't that important. But the lowest case is not currently possible (unless you're willing to make the whole graph lowest). But in general, it would be nice if this tuning of priorities happened automatically and at a much more granular level.

I propose that for all tasks that are in the optimized graph but not the target graph, we set the priority based on the following algorithm:

min(<highest allowed priority>, max(<priorities of dependent tasks>))

Obtaining the <highest allowed priority> would require a request to the Taskcluster auth service. I'm not sure if we'd want the ability to toggle this behaviour on or off. I'd be inclined to make it the default at the very least.