0.22.x scheduler driver drops 0.23.x reconciliation status updates due to missing StatusUpdate.uuid.

In the process of fixing MESOS-2940, we accidentally introduced a non-backwards compatible change:

--> StatusUpdate.uuid was required in 0.22.x and was always set.
--> StatusUpdate.uuid is optional in 0.23.x and the master is not setting it for master-generated updates.

In 0.22.x, the scheduler driver ignores the 'uuid' for master/driver generated updates already. I'd suggest the following fix:

1. In 0.23.x, rather than not setting StatusUpdate.uuid, set it to an empty string.
2. In 0.23.x, ensure the scheduler driver also ignores empty StatusUpdate.uuids.
3. In 0.24.x, stop setting StatusUpdate.uuid.