fix(JobWatcher): offload scheduling to Scheduler
refactor(Agent): restyle doServiceHeartbeat
fix(agent): stop local jobs that are not scheduled
refactor(Agent): restyle doMachineHeartbeat
fix(list-units): fix nil-pointer reference bug

If the Machine is nil on the JobState object given to the list-units
command, corectl will panic. This is masking a larger problem, but
the panic is not helpful at this level. Handle the nil-pointer for
now so we do not screw users.
refactor(JobWatcher): Use time.Tick rather than time.Sleep
chore(JobWatcher): expand and tidy up logging
fix(locks): Add prefix to lock for human-readibility
fix(registry): Robustify Registry.AcquireLock and call it only through Claim* methods
refactor(JobWatcher): Add moar logging
fix(JobWatcher): Fix nil-pointer reference
refactor(JobWatch): Refactor how JobWatch objects are stored

* Move JobWatch objects from coreos.com/coreinit/watch/<name> to
  coreos.com/coreinit/watch/<name>/object
* Place JobWatch lock at coreos.com/coreinit/watch/<name>/lock