Add job benchmark loop

[Kobzol]

This PR adds the main logic required to execute job benchmarks in the collector.

It handles:

• Updating the collector's heartbeat periodically
• Quick loading of already downloaded sysroots, to avoid redownloading them in-between jobs and also collector restarts (useful for local testing)
• Dequeing jobs, including in-progress jobs and expanding benchmark sets
• Distinguishing between transient and permanent job errors, storing job errors into the DB, and marking jobs as failed or successful
• Marking jobs that have been dequeued too many times as failed
• Reconnecting to the DB if a transient I/O/network/DB error happens, to try to refresh the DB connection

[Jamesbarford]

LGTM 👍

[Kobzol]

Ok, let's try.