The goal of this job queue design is to process jobs in order, with each job
moving through a pipeline of tasks that need to be completed.

At any given time, a job is in one of a few given states, e.g. "queued". These
states are explained below. Along with this, each job also has a "dispatched"
flag. If this flag is set to true, it means this job is currently being
processed. "Being processed" could mean anything; it depends entirely on the
state a job's in. While a job is dispatched, it is no longer present in the
priority queue of its respective state.

## Job

A job describes a scheduled build as it moves through the pipeline of states.
The job queue datastructure keeps track of all jobs in a central red-black
binary tree. For each state, a priority queue tracks in what order jobs should
be processed.

## States

* `queued`: a job that's in the job queue but does not yet need to be executed
  (as defined by its timestamp)
* `ready`: a job that's scheduled for building, with all preprocessing tasks
  fulfilled.
* `build_finished`: a job whose build has finished, and is waiting for any
  post-build tasks.
* `failed`: a job whose processing failed at some point. Jobs in this state
  include a failure report that describes in what state they failed, and why.