Renew event ID when event object is reused (close #390)

[AlexBenny]

The event ID is set in the event constructor. We want the event ID set when the event is tracked. Unless the user has set it programmatically.

[coveralls]

Coverage decreased (-0.5%) to 75.06% when pulling 2941298 on issue/390-renew_eventid_when_reused into d8ea489 on release/1.5.0.

[paulboocock]

Curious about the new depracted methods. Other than that this looks good and I'll approve once I understand a little more about those.

[paulboocock]

I'm not sure why we have introduced a side effect in this function and then made a new function to get the unchanged version. Whats the plan for v2?

[paulboocock]

It also seems odd to introduce deprecated methods...

[paulboocock]

Same comments here as above.

[AlexBenny]

Thanks @paulboocock , all these comments are related to a design issue.
Normally the event object has a timestamp and an eventId assigned. That can cause duplications if the same object is reused multiple times.
I moved the eventId and timestamp assignment into the tracker rather than the event constructor.
Unfortunately, these are public API so I can't change the contract of the getEventId and getDeviceCreatedTimestamp. They have to return a non-null value and I have to let the user set them as usual if they want (until v.2.0).

In brief:

• the side effect exists because I can't change the behaviour of the two old get methods
• the two new methods return the "actual" values because Event is an interface and I can access eventId and timestamp only by methods (no properties on interfaces)
• everything is deprecated because the v.2.0 of Event interface will not have any trace of eventID and timestamp, so no need of the old and new methods!

[paulboocock]

Ok, that's a good enough excuse for me. 2.0.0 can't come soon enough :-)

LGTM.