set maximum concurrency (& priority)?

[mbernerslee]

be able to set maximum concurrency somehow, such that if max_concurrency = 2, and 5 build steps want to run at once, only 2 will run... and the other 3 are queued behind (but only the next two will run, and so on)

additionally be able set the "priority" of a build step, such that if max_concurrency = 2, and 5 want to run at once, my two highest priority build steps will run first.

(it could be implicitly determined by the order in which they appear in the JSON, but that seems too implicit & magical)