Return eventId from Tracker.track()

[mscwilson]

The event ID (a UUID string) is generated when an Event object is created. We also currently provide the option for the user to override this and manually set an event ID.

This is risky and could result in events not having unique IDs. This functionality should be deprecated.

[mscwilson]

It's also possible to manually set the - i.e. override the autogenerated - deviceCreatedTimestamp. But trueTimestamp is provided for manual timestamp setting already? Should users be able to set dtm?

[paulboocock]

It should not be possible to manually set the deviceCreatedTimestamp, this should come from the system clock. True Timestamp is available if a custom timestamp needs to be provided.

[mscwilson]

I've changed the title of this issue to reflect the more interesting part of the changes I've made in PR #310.