adds support for builder priority queues

[jerstern]

Change Description

On every commit, we run a large number of tests across many instances simultaneously. This allows us to parallelize the previously non parallelized tests that competed for resources on the same builder/instance. The primary effect is a much faster build than we've typically known. One side effect is that, in the case of many scheduled builds running concurrently, build steps queue up and waits occur for some builds (regardless of whether the builds consuming all the instances are scheduled).

This change does two things:

1. Moves scheduled builds to a separate queue that's throttled aggressively (8 instances max for linux and 2 instances max for mac)
2. Introduces a new builder queue that scales to 4x the current number of builders in the fleet

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[zreyn]

🚀