fleet engine constantly running

The fleet engine is triggered at a 2s interval, but also by any events generated by an etcd watch on the /_coreos.com/fleet/ namespace. Given the amount of heartbeating going on (machines, jobs, states, etc) that doesn't represent actual changes, this causes the engine to be triggered constantly.

We should figure out a way to only trigger the engine when an event represents an actual change that the to-which the engine could respond.