Use nil event to mark start of watchHost routine

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Don't leave a Mutex locked!

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Ensure scheduler tracks jobs before they actually start

This avoids races where host start events are received before the job
has been added to the scheduler's jobs (in which case the event would be
ignored)

Signed-off-by: Lewis Marshall <lewis@lmars.net>