Store and prioritize jobs

[maleadt]

We should be able to re-start the Nanosoldier server without losing submitted (or even running) jobs, by serializing them to disk. In addition, when touching this code we should figure out a way to prioritize certain jobs, e.g., dailies should run when there's no requested jobs, release-related jobs should always run first, and I guess we should maybe prioritize short jobs too.