fixes #609 - Don't store status updates when state and health didn't cha...

...nge
It looks like this method will return false for interesting items in the following sequence of StatusUpdate messages with state TASK_RUNNING:

- `{ "state": "TASK_RUNNING" }` (first enters running state)
- `{ "state": "TASK_RUNNING", "healthy": true }` (return `false`, status not written)
- `{ "state": "TASK_RUNNING" }` (reconciled task status)
- `{ "state": "TASK_RUNNING", "healthy": false }` (return `false`, status not written)
- ...

Should this be something like the following?

```scala
statusB.hasHealthy &&
  (!statusA.hasHealthy || statusA.getHealthy != statusB.getHealthy)
```