Switch to Machine IDs

1. Dropped the boot_id config option, nobody was using it
2. Renamed X-ConditionMachineBootID to X-ConditionMachineID thinking nobody was using it
3. The ID fields on MachineState, JobOffer, and JobBid have not changed, so the data models don't actually have backwards-incompatible changes other than the data in them is now pulled from a different source.