Implement DataScheduler

[JanC]

When a triggered DataPayload has an executeAt or delayFor value, the SDK needs to schedule that execution.
The responsibility of the DataScheduler is to take care of the fire dates and simply report back when execution date is reached.

Possible implementations:

• We simply pass the internal uuid of the persisted DataPayload and the simply pass this uuid in a callback
• The caller passes a closure that is called