Kill util::jobs framework

[rphmeier]

It did not anticipate things like #3779, where subsystems would need to have "jobs" communicate or coordinate between each other. This could be done in a few parts, where we'd first migrate over the subsystems which use the framework, and then remove the framework entirely. It's not particularly useful code, it adds 'magic' to subsystems, and constitutes an extra maintenance burden.

• Backing
• bitfield-signing
• provisioner
• remove all JobSubsystem code